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OUTOKUMPU OY, Outokumpu
773947
Method for controlling the heat content and evening out
temperatures in various sulfidizing processes
,
The present invention relates to the control of various
sulfidizing processes. The invention relates primarily to the
control of sulfidizing processes in which the object is a
sulfidizing treatment, mainly in solid state, of polymorphous
complex minerals which contain metals Cu, Ni, Co, Zn, Pb and
Fe as principal components and also metals which constitute
impurities with regard to these. The purpose of the sulf;dizing
is in this case the decomposition of the original complex
minerals and the simultaneous conversion of their principal
metals and impurities to stable, independent sulfides. Such a
conversion based on sulfidizing requires a sufficient reaction
velocity and, in order to produce stable sulfides usually with ?
a high sulfur content, the use of high partial pressures of
sulfur. On the other hand, many complex minerals melt at very
low temperatures, and therefore the sulfidizing must be carried
out under conditions in which the melting points of the mineral
matrices are not surpassed and/or ~n which the use of high sulfur
pressures raises the temnerature ranges of the breaking-down
and rearrangement of mineral matrices. The sulfidizing
processes of the said type are usually performed within the
.
temperature ran~e 700-1200K (427-927C) and within the sulfur
vapor pressure range 0.1-1.0 atm.
When using high sulfur pressures, the heat required by the
process must be introduced into the system indirectly,since
the use of, for example, a fossil fuel in the reaction space
causes the partial pressure of sulfur to decrease, and since
the combustion gas reactions cause losses of sulfur. The in~
direct heating of the sulfidizing systems for its part easily
causes the complex minerals to melt and sinter since, owing to
the poor heat transfer caPacities of mineral powders, a large
temperature gradient must be used in the transfer.
The method according to the invention re~ders unnecessary both
the external heat transfer of sulfidizing systems and the internaL
use of fossil fuel.
The method according to the invention can also be used in
high-temperature sulfidizing processes, melt refining, etc.
The method according to the invention is to a high degree
independent of the equipment and other technology used in the
process. By the new method, the elemental sulfur can easily be
cycled, no polluting gases are produced, and both the solid and
the gaseous phases are obtained as powders or in forms which
condense as sulfur polymers. Therefore, in many sulfidizing
processes the new method is not only economical as regards
energy but also almost indispensable in terms of both safety
at work and environmental protection.
,~
In the process a~ccording to USP 3,459,535 the solubility of
the copper present in Cu-Fe-S minerals is improved in an acid,
oxidizing leach in an autoclave. This is performed by treating
the said minerals within the temperature range 300-600C in
contact with elemental sulfur and its vapor. The treatment is
performed in an externally heated retort. It can be shown that
below 501C chalcopyrite and bornite sulfidize to idaite
(Cu5FeS6) and pyrite. Above 501C, bornite again becomes stable,
and above 508C, both bornite and chalcopyrite are stable.
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The process according to USP 3,817,743 is analogous to the
above. In this process the chalcopyrite is sulfidized within the
temperature range 460-500C to X-bornite (Cu3FeS2 phase in
which there is an excess of sulfur, about 0.5% by weight) and
to idaite at a sulfur pressure of PS = 200 mmHg.
Processes older than those described above are represented by
that according to USP 1,523,980 (Colcord or Hulst process), in
which the copper present as an impurity in molten lead is
removed by sulfidizing it as solid digenite by means of (solid
or vapor) elemental sulfur. In the process according to DRP
497,312, sulfur or sulfur-yielding material is added to molten
metal or a molten metal alloy (an alloy of Pb, Sn, Sb, etc.) in
order to remove the copper in sulfide form. The reducing agents
used in the process are pitch, tar, etc. In the process according
to DRP 431,984, impurity metals, Zn, Pb, Cu, etc.,are removed
as sulfides from raw antimony by adding sulfur and by smelting.
The use of elemental sulfur vapor for the determination of phase-
equilibriums is a commonly used, although difficult, method.
Various applications of the process are described in the followin3
publications: H.E. Marwin, R.H. Lombard: Econ. Geol., 32, 1937,
203-284; T. Rosenqvist, T. Harvig: ~eddelelse Nr. 12 fra
Metallurgisk komité, Trondheim, Norway, May 1958. 21-52.
The objective of reduction-sulfidization processes is to reduce
oxide ores at a low temperature and to recover, either as metal
alloys or sulfide matte, the valuable metals present in them in
low concentrations. This group includes numerous processes for
the refining of limonitic and/or serpentinitic laterite ores which
contain Ni, Co and Cu. The valuable metals are reduced and
sulfidized out from the ores, and the obtained product i9
separated from the gangue by conventional methods ~leaching,
magnetic separation, smelting, etc.). The reduction and the
sulfidization are performed within the temperature range 700-1100C
using a tubular furnace or a fluid-bed reactor (e.g. USP 3,004,846;
3,030,201; 3,272,616) and the sulfidizing agent is usually the
reduction qas (H2S, COS, etc.). The sulfidization can in this ~ -
case be performed using many sul~ur-yielding materials, pyrites,
injection of sulfur into the combustion gases, SO2 fed to the
reduction zones, S-bearing raw oil, etc. (e.g. USP 3,388,870;
3,535,105; 3.772,423; 3,819,801).
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What the said sulfidizing processes have in common is that, in
aiminq at producing as a solid or a molten phase a metallized or
conventional sulfide matte, the sulfur pressure required for
the sulfidization is usually very low. The following sulfidizing
reactions of nickel are mentioned as examnles:
NiO ~ Ni ~ Ni ~ Ni3S2 + x 700C PS2 ~ 10 -3.7x10 atm.
matte: 20.6-21.8 S by weight;900C: Ps2 ~ 1.5x10 8_10 2 atm.
matte: 19.2-30.3 S by weight.
The cate~ory of processes in which the aim is to vaporize the
impurities of concentrates and ores or to convert them to an
easily separable form includes, as does the former one, a large
number of patented and/or otherwise published methods. In the
process according to USP 1,762,~67 the impurity components are
removed by fractional distillation from primarily complex
concentrates of copper and from technical intermediate products
in such a manner that the sulfides of the metals Hg, As, Sb,
Sn, Cd are vaporized, within the temperature range 600-1200C,
and finally the sulfide of zinc is vaporized, the remaining
product phase being a Cu-Fe sulfide matte. When the process is
carried out in, for example, a tubular furnace, sulfur or
sulfur-yielding compounds, iron, and lime (to eliminate the
effects of the low melting point of sulfides) are, when necessary
added to the batch in àddition to coke. The batch is heated by
means of combustion gases flowing countercurrently to it. Using
these gases the volatile sulfides can be oxidized before they
are discharged from the furnace.
The process according to DRP 504,487 (Skappel) for the
treatment of complex ores is very well known. In one embodlment
of the process, impure concentrates or industrial intermediate
products are smelted or sintered together with sodium sulfide
(Na2$, Na2SO4 + C, etc.), whereby suitable eutectics and Na2S-
MeS binary salts are produced. The product obtained is broken up
~ .
with a flow of water to produce a powder-slurry mixture
containing separate sulfides, and the separate sulfide phases
can be recovered from this mixture by conventional methods.
When the batch is heated or smelted, a portion of the impurities
vaporizes and a portion is removed during the aqueous wash stage
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(e.q. As). The temperature ran~e of the application of the
process is hetween 600C and 1600C, depending on the raw
material.
In the process according to Finnish Patent 49 186, serpentinic
laterites are subjected to a reduction-sulfidization at a low
tem~erature. The metallized sulfide produced by sulfidization
is "filtration-smelted" by exploiting the melt solubility gap
of the Me-MeS system to form sufficiently lar~e particles (the
oxide matrix remaining solid). sy long annealing performed at
a temperature decreasin~ from the ran~e of the solubility ~ap
the metals of the Me-MeS system are divided in such a manner that
Ni, Co, Pt-metals, (Fe), etc., are concentrated in the metal
phase and metals Cu, Cr, Mn, etc., are concentrated in the
sulfide layer surrounding this phase. The Me-MeS system is
separated from the oxide matrix of the product, and the Me phase
is separated from the MeS phase, both by conventional methods.
In the above processes the heat required by the process
is introduced into the batch by means of fossil fuel in the
reaction space. irhereby the sulfur potential of the system
remains low.
Ca~a~/~a~ ~af~n~ /~0 IJ ~S~J5/o
In the process according to F~ h ~atcnt ~pplication l~l~/74,
the components of complex concentrates are rearranged from their
complex compounds into stable, independent sulfides corresponding
to the new conditions, both as re~ards the principal metals
(Cu, Ni, Co, Pb, Zn) and the impurities (As, Sb, Bi, Se, Te,
Ga, In, Te, Hg, Ge, Sn, etc.). Some of the impurities vaporize
quantitatively under the treatment conditions (600-800C,
Ps = 0.25-l.00 atm) and some can be separated by conventional
means after the processin~. Usually the process does not require
the introduction of additional heat into the sulfidization
furnace. Another example of processes operating with external
additional heat is the process, which is still subject to much
research, in which arsenic, for example, is separated from its
compounds by heating them (Fe-, Cu-, Ni-, Co-based sulfldes or
arsenides) to~ether with pyrite. However, the quantities of
pyrite required are lar~e. The decomposition of the pyrite in
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order to make sulfur available for the reactions also requires large amounts
of energy (e.g. Il-W. Loose: Chemismus der Entfernung von Arsen aus seinen
Verbindungen mit Eisen, Kupfer, Nickel und Kobalt durch Erhitzen in Anwesen-
heit von Pyrit, Dissertation Arbeit, Breslau 1931, 1-73).
According to the present invention there is provided a method for
controlling the heat content and evening out the temperatures in sulfidizing
processes, in which the sulfidizing is performed in a sulfur atmosphere, com-
prising regulating the partial pressure of sulfur in the sulfur atmosphere
in order to utilize the energy of dissociation and recombination of sulfur
molecules, the temperature of the sulfur atmosphere being 400-900C and the
partial pressure of sulfur being 0.1-1 atm.
The control method according to the invention utilizes the very high
dissociation-recombination energies linked with changes in the atomic number
of sulfur-vapor molecules within the temperature and pressure ranges specified
below. These amounts of energy which are freed or bound are functions of
both the temperature and the partial pressure of sulfur vapor.
The amounts of dissociation-recombination energy are so large that
in many low-temperature sulfidizing processes they suffice to cover the
amounts of energy required for the sulfidizing reactions, impurity vaporiz-
ation, and the heat losses of the apparatus. Thus the sulfidizing processescan be made self-sufficient in terms of heat economy by regulating only the
temperature and/or partial pressure of the sulfur vapor to be fed into the
sulfidizing system.
The new method for regulating and controlling sulfidizing processes
operates within the temperature range 400-900C and within the elemental
sulfu~ vapQr pressure range 0.1-1.00 atm.
The invention relates primarily to the treatment of predominantly
sulfidic complex ores which contain, as their principal metal, copper, zinc
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lead, nickel, cobalt and iron and which have been produced by low-temperature
and low-pressure pneumatolytic and especially hydrothermal minerali~ation.
The technological utili~ation of these ores is effectively prevented by a
large quantity o heavy elements usually present as impurities in these ores.
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One of the things common to these complex minerals is that they
are both ~ormed and dissociated and/or melt at very low
temperatures. In order to separate the sulfides of both the
impurities and the principal components from each other to form
separate phases, the sulfidization of these minerals must be
controlled as regards the order of phenomena, velocity, degree
of sulfidization and other factors. This must be done in such
a manner that there are hardly any molten phases formed.
The method according to the invention makes it ~ossible to
perform the sulfidization process so that it is self-sufficient
as regards heat. In this case the heat yielded by the actual
exothermal mineral sulfidization must be sufficient to cover the
heat losses of the processing apparatus, the vaporization
enthalpies, and the heat required for heating the sulfidizing
agent and the concentrate from the pre-heating temperature to
the reaction temperature. Thus no additional heat is fed into
the sulfidizing system by using fossil fuel in the reaction
chamber or by heating the processing equipment externally.
The feeding of such additional heat would usually be very
difficult to implement because of the low melting-temperature
range of the material being processed or because of the high
sulfur potential required.
In the new method under discussion the realization of the thermal
- balance of the sulfidizing system is regulated as follows:
the ore is pre-heated to a temperature below that at which any
chemical or physical changes detrimental to the process take
place, and the amounts of heat are regulated, taking the
exothermal nature of sulfidizing processes into account, by
means of the considerable dissociation-recombination heat amounts
linked with the change in the atomic number of the sulfur vapor
molecules. These amounts of heat are exploited by regulating
the feeding temperature and partial pressure of sulfur vapor
and the point at which the sulfur vapor and other components
are essentially fed into the system.
By the method according to the invention it is possible, in
sulfidizing processes, to control not only the heat balance
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of the system but also the melting temperature ranges of complex
minerals, the velocity of the sulfidizing reactions, the structurs
of the product matrix, and the structures of both the impurity
sulfides remaining in the matrix and the impurity compounds
vaporizing under the conditions o the process. The method
makes it possible to use a more versatile and simultaneously less
complicated sulfidizing technique, which is hiqhly economical
and usually independent of the technical equipment used. The
method also makes it possible to carry out sulfidizing processes
which require high sulfur potentials and have not been previously
feasible.
By the method according to the invention, various sulfide
processes are thus regulated by utilizing the great changes in
the heat content of the gas phase in connection with the
dissociation and recombination of molecules. The following is
an example of the proportions of the heat amounts: At a
pressure of one atmosphere, at the temperature T = 700K,
the average atomic number o~ the molecules of sulfur vapor is
V = 6.97 and its enthalpy values are ~kcal/kg S): ~as
enthalpy ~H700_298 = 62.82 and heat of formation ~H298 =
170.36. At the temperature T = 1100K the respective values
' 1100-298 = 96.58 and ~H298 = 575 14 Thus
within the temperature range involved,the heat available for
process control, ~(~He+f) = 404.78 kcal/kg S. In this case the
proportion of conventional thermal gas enthalpy is only 8.3~ -
and the proportion of dissociation-recombination energy 91.7%.
This dissociation ener~y is not only a function of the tempera- ~.
ture but also a function of the partial pressure of sulfur
vapor and in such a manner that the change in energy corresponding
to a chan~e in the atomic number of the molecules occurs at
different temperature ran~es, depending on the different partial
; pressures. The suitability of sulfur vapor for the control of
sulfidizin~ processes thus has a very wide range of operation,
`~ especially in processes in which the partial pressure of sulfur
i~ vapor varies within the range Ps = 0.1-1.00 atm and within
;~ ~ the temperature range T = 700-1200K. Utilizing the peculiarbehavior oif sulfur vapor it is possible to control not only the
temperature of the processes but also the final states,
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composition and cooling of the reaction products, the order
of the sulfidizing phenomena, eXcit~tign, etc.
The method according to the invention is especially suitable
for performing sulfidizing processes which re~uire high partial
pressures of elemental sulfur. Because of the poor heat
conductivity of the concentrate and ore powders and because of
the melting of the complexes, indirect transfer of external heat
is usually difficult in the processes involved. On the other hand
direct heating of the reaction chamber using, for example,
combustion gases, is not suitable because of the resulting
decrease in th~ partial pressure of sulfur and the formation of
detrimental, sulfur-consuming gas components (COS, H2S, etc.).
The use of combustion cases in the processes would also result
in pollutant gases technically difficult to handle, as well
as in dust problems with increasing amounts of gas. It is evident
that the method is also applicable to many high-temperature
sulfidizing processes.
Some sulfidizing processes falling within the sphere of the
method accordin~ to the invention are listed briefly below:
.
- The br~ d~wn of impure complex ores and the ~rearrangement
Ca~al/~7 P~l*~7f QJo . /~ ~S;~
D of the matrix (e.g. innish Patont .~p~lication Mo lgl2,~7l).
- The sulfidization of impurities to separate nonvolatile
minerals (partial-melt processes) and multi-stage sulfidizing
processes (Examples 4 and 5).
- Low-temperàture sulfidizi~g processes in which the preheating,
ignition, and extinguishing of the concentrate are carried out
with elemental sulfur (Examples 1-3).
- Colcord and similar processes (USP 1,523,980).
,
- The refining of antimony and tin alloys (DRP 431,984, British
Patent 1,348,278).
- Constant-temperature sulfidizing processes.
- Matrix changes of copper concentrates prior to leaching in
continuous processes (USP 3,459,535).
- High-temperature zone reduction-sulfidization processes, in
the control of zone temperatures (USP 3,754,891; 3,900,310).
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The method accordin~ to the invention is highly aoplicable
-to the treatment o~ predoninantly sulfidic complex ores which
contain as their urincipal metals Cu, Zn, Pb, Ni, Co, Fe, etc.,
and which have been formed partly through pegmatitic-pneumatolyti~
and esoecially hydrothermal mineralization. Hence, heavy elements
representin~ the high ordinal numbers of the periodic system,
which are very mobile and have high vapor pressures within the
range of operation o the process (but are to be considered
harmful impurities as regards the principal metals) have
concentrated in the mineral-forming "solutions" (temperature
~400-450C, pressure 225-250 kg/cm2). The structure of such ore
minerals can be altered by a suitable sulfidizing treatment so
that the impurities and the principal metals form separate,
independent sulfides which can be separated ~rom each other by
conventional methods.
The following include some of the mineral categories involved,
grouped according to their composition: -
Pyritic and arsenic- and antimony-rich groups:
(Fe,Co,Ni)(S,Se,Te,As)2, (Fe,Co,Ni)(As,Sb)S; Cu(Fe,Ga,In)S2,
Cu3(Ge,Fe,As,Sb)S4,etc. -~
Lead, zinc and silver groups:
(Cu~Ag)20(Fe~Zn~Hg~Ge,Sn)4(As,Sb~Bi)8S26, (Zn,Cd,Hg)(S,Se,Te),
Pb(S,Se,Te).
,.
Tin, zinc and silver groups:
Cu3(As,Sb,Fe,Ge,V)S4, Cu2(Ag,Fe,Zn,Sn)S4.
Cobalt, nickel, silver, bismuth, and uranium groups:
(Co,Ni,Ag,U)(As,Bi)3.
. . :
Arsenic, antimony and bismuth complex minerals:
Ag3(As,Sb)S3, Cu3NiS3, Cu(Sb,Bi)S2, A~(As,Bi)S2, (Pb,Cu)(As,Sb,
Bi)S3, etc.
. .
In addition to natural minerals, many high-temperature
technical intermediate products which contain respective
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impurities but are not complex also fall within the sphere of
the method according to the invention.
The control method according to the invention is thus based on
the exploitation of the dissociation-recombination energy of
molecules in connection with the change in the atomic number
of sulfur vapor molecules as a function of the partial pressure
and the temperature.
It is known that sulfur vapor has a very complex structure.
The detailed structure of individual vapor molecules is almost
unknown. Also, the quantitative proportions of the different
vapor molecules in the gas phase are under dispute. The heats
of formation of the categories of molecules, as well as their
specific heats, are at least technically known with sufficient
precision, and so the total enthalpies of the separate components
of the gas phase can be calculated with relatively g~eat
precision.
The invention is described below in more detail with reference
to the accompanying drawings. Fi~ures lA and lB in the drawings
depict the molar proportions and enthalpy of the sulfur vapor
as functions of temperature. Figure 2 depicts the total pressure
of sulfur vapor as a function of the average atomic number of
its vapor molecules and the temperature, Figure 3 depicts
part of the apparatus developed for applying the method according
to the invention, Figure 4 depicts a side elevation of the
entire apparatus, and Figure 5 depicts an equilibrium diagram ~ -
of the minerals as a function of the sulfur activity and the
temperature.
Figure lA shows the molar proportions of sulfur vapor Sv
(v= 2-8~ as a function of the temperature. Of special note is
that the sulfur molecule S4,used in conventional calculations,
is almost completely absent in the ga~s phase. Figure lB shows
the calculated total enthalpy of sulfur vapor (in comparison
with solid rhombic sulfur3, corresponding to vapor pressures Ps =
1.0 and 0.1 atm. The average change in the atomic number x
of the gas phase molecules as a function of the temperature
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is also indicated in the figure. The figure also shows, as a
parameter, the change in the total vapor pressure at different
partial pressures of vapor dissociated to a molecule of atomic
number two.
The dissociation energy used for the control of the sulfidization
is seen qualitatively in Figure lB. At 1100 K (827C) the total
heat of sulfur vapor at one atmosphere is ~He+f = 575.1 kcal/kg. ~ -
The average atomic number of the vapor molecules in this case
is v = 2.07.
When the gas phase cools to a temperature of 700K (427C),
a heat amount of 404.7 kcal/kg is released by the recombination
of the sulfur molecules, while the sulfur remains in vapor
state (700K, ~He+f = 170.4 kcal/kg, v = 6.97~. The released ~
heat is thus 240% of the heat required for smelting the sulfur -
and for vaporizing it to 700K. As can be seen f~om Figure lB, ~-
the partial pressure of sulfur vapor has an important effect
on the temperature range of the dissociation-recombination
energy.
The amount of reIeased or bound energy within à wide temperature
and partial~pressure range of sulfur vapor allows a versatile
technical use of the phenomenon in many sulfidizing processes,
especially at low temperatures.
For example, the approximate isobaric equations of the following -
form can be caiculated for the sulfur vapor enthalpy change as
a function of the temperature:
S = 1.0 atm, T = 800-1000K, ~He+f, kcal/kg S =
exp10(-159.568 + 184.577 x 10 3T - 69.002 x 10 T + 46727~T)
PS = 0.1 atm, T = 700-900K, ~He+f, kcal/kg S =
exp10(-198.922 + 257.952 x 10 3T - 108.538 x 10 6T2 + 51673/T).
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Similar approximate equations can be calculated for all partial
pressures by determining the gas composition. It should be
noted that the equation in itself does not exactly correspond
to reality, since in the actual process the consumption of
sulfur results in a change in its partial pressure, a fact
which is not in itself taken into consideration in the equation.
Dissociation of sulfur vapor by thermal excitation: -
The mutual balancing between the different types of molecules
of sulfur vapor is a very slow process, especially at low
temperatures. In the balancing, sulfur represents an intermediate
case between or~anic and inorganic compounds. The vaporization
temperature of sulfur and its heat of vaporization are lower
than those of other inorganic materials and yet only a little
hi~her than those of most organic compounds (sublimation
energies, ~H298, kcal/g at: 2.87-2.97/S, 19.2/Te, 29.0/As,
49.3/Sb, etc.). Most organic compounds, including their vap,ors,
are thermodynamically metastable. The slow settling of the
equilibrium is obviously another reason for some of the many
allotropes of sulfur.
,~
The activation energy of the balancing of sulfur vapor is in
proportion to the energ~ of the S-S bond in ring molecules,
a form in which all sulfur molecules appear to be present. All
molecules of sulfur apparently have a double bond. The
dissociation ener~y of molecules increases only slightly with
increased atomic number: D298, kcal/atom: 50.8S/S2, 54.40/53,
54.73jS4, 60.56/S5, 61.90/S6, 62.31/S7 and 63.06/S8. Selenium,
which is in many respects analogous to sulfur, does not show
conditions of inequilibrium at low temperatures. Its bond energy
and its vaporization temperature show, respectively, lower and
higher values than those of sulfur.
Owing to the behavior of sulfur vapor, it is advan*ageous to
produce this vapor at a lowered pressure ahd using a suitable
catalyst. In the present method, sulfur vapor is produced
using a carrier ga5, which is saturated in molten sulfur and
directed to a preheatin~ apparatus.
,~
The vapor pressure of molten sulfur is of the form:
Log ~P~atm) = -6109.6411/T + 16.64157
-17.05358 x 10 3T + 7.9769 x 10 6T2
The partial pressure of the sulfur vapor obtained is regulated
by controlling the temperature of the molten sulfur bath. For example, when
a temperature of 427C is used for the sulfur bath, the partial pressure of
the vapor obtained is PS = o~757 atm a~d the average atomic number of its
molecules corresponds to v = 6.62. Thus the following gas phase is obtained
from the pre-heating apparatus at 600C: PS = 0.845 atm, v = 4.o5, and at
lo 700C: PS = 0.891 atm, v = 2.65.
A ther.~ally stable general sulfur catalyst is a suitable equilibr-
ium catalyst of sulfur vapor. A chromite catalyst is especially appropriate
for this purpose slnce in addition to its excellent catalytic capacity it
has good heat conductivity, thermal stability and, compared with other cat-
alysts, a small but effective surface area per unit weight (the space require-
ment of the catalyst is small since its denslty is high).
Figure 3 shows an apparatus for sulfur vapor production used in
the new process. me apparatus works under atmospheric pressure. me vapor-
izer 2 consists of two electricaIly-heated tanks, one inside the other. The
;20 temperature of the outer tank 15 provided With a stirrer 23 is far below the
vaporization point of sulfur, and so elemental sulfur can be fed into this
tank throu~h inlet 24 in solid state. ~he inner tank 16 is an electric
vaporizer, thermally insulated on the outside. It forms a oommunicating
vessel with the outer tank 15. m e sulfur 22 is vaporized and carried out
of the tank 16 by means of pre-heated nitrogen 4. The sulfur vapor and the
carrier gas flow from the vaporizer into an electric pre-heater 3 filled
with a chrcmite catalyst. In the pre-heater 3 the sulfur vapor is brought
to the desired tem~erature and the respective e~uilibrium. From the pre-
- 14 -
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heater the vapor flows along an electrioally-heated pipe into the reaction
chamber of the sulfidizing apparatus 1, into which the pre-heated concentrate
is also fed through inlet 25.
Figure 4 shows one embodiment of the apparatus for oarrying out
suLfid:Lzing processes. The system comprises a concentrate pre-heating device
6 with its feeding and heating devices 7, a sulfidizing devioe 1 with its
feeding and discharging devices ~sulifidized product concentrate 8, gas phase
9, suLfur polymer 10) a vaporizer 2 corresponding to Figure 3, a vapor dis-
sociating pre-heater 3, and nitrogen vaporizer 5.
Utilization of the thenmal dissociation-recombination energy of
sulfur vapor for controlling sulfidizing processes:
~he new control method is applicable primarily to sulfidizing pro-
cesses which are carried out at a high partial pressure of sulfur vapor and
often at low temperature. mese processes are usually very complicated as
regards their mechanism and the technological operations required. For this
reason the examples to be discussed contain essential pa~sages relating to
the theoretical grounds of the control method. me examples cover only part
of the sphere of application of the new method. It should also be noted that
some of the sulfidizing processes falling within the sphere of application
of the method, perforned using a high sul~ur pressure, are currently imposs-
ible to oarry out by other procedures, and therefore the control method is
crucial, being almost the only method for controlling the prooesses.
To illustrate the control method Or the sulfidizing processes under
discusslon, the stability ranges of the initial and product materials and
the gas phases of the embodiments given as examples have been calculated as
a function of the temperature and the sulfur activity and are shown in Figure
5. The aotion of the sulfur processes in reality can be observed from the
temperature and sulfur pressure values given in the examples.
- 15 -
-.
, - ,
.
:
~132S~3
:
F~2mple 1
This example illustrates the processin~ of a sulfidic ~ -
cobalt arsenide concentr~te. In this process the complex mineral
present in the concentrate is decomposed and sulfidized to a
compound more stable than previously. The arsenic present in
: .
. ~
1~ ~
15a -
. . ~ , . , ~ .
^,: . -, , ~. . - :
. .. .
16
Z58
the concentrate leaves the system together with sulfur vapor.
Thus the partial pressure of sulfur in the vapor phase must be
sufficient both to maintain the stability of the volatile
arsenic sulfide and to maintain the mineral composition of the
sulfide concentrate obtained as a product close to that
correspondin~ to the mineral equilibrium: (Co, Fe)S2 + FelOS12.
According to the equilibrium diagram of Figure S, at 1000K
(727C) the partial pressure of sulfur vapor, both before and
after the sulfidization, must be over 0.1 atm (stability range
COS2-FeSl+Xi As4S6(g) boils a
The calculated heat balance and balance of materials
corresponding to Example 1, the bases of calculations used, and
the explanations are compiled in Table 1. The partial pressure
of sulfur vapor in the feed gas phase was PS = 0 50 atm and that
in the product gas phase, respectively, PS = 0.255 atm and so,
according to the equilibrium diagram, the system is within the
correct range of operation both before and after the sulfidiza-
t on ( roduct gas phase, atm: PS2 = 0.255, PAs4S6 0.064,
The temperature of the suIfur bath in the vaporization of sulfur
was 630K and the average atomic number of the vapor molecules
was v = 6.88. The total enthalpy of sulfur vapor at the
vaporization temperature was 160.1 kcal/~g S. The saturation
pressure of sulfur vapor was PS = 0.233 atm, i.e. PS2 = 0.50
atm, calculated as the S2 pressure of the equilibrium diagram.
The heat of formation of the concentrate corresponding to
Example 2 was AH298 = -259.8 Mcal/t and that of the sulfidiæed
concentrate respectively ~H298 = -337.3 Mcal/t. The decomposition
and rearrangement of the complex concentrate is thus a strongly
exothermal process in the case under discussion. The heat-
consuming factors in the process are the heating of the con-
centrate and the sulfur vapor (and the carrier gas) from the feed
temperature to the reaction temperature and the heat losses o
the processing apparatus. It can be seen from the balances of
Table 1 that when the concentrate is fed into the system at
the temperature Tl = 473K (200C), the feed temperature of
the gas phase must be T2 = 950K (677C). When the pre-heating
.... , ,. , .: . : ~'' . '
.-
,, . ~ -
17 ~h3Z5~d
temperature of the concentrate is raised to the value T1 =
773K (500C), the gas phase must be fed into the system at
T2 = 869K (596C). In each case the -total enthalpy of the
sulfur vapor is still far below the value corresponding to the
reaction temperature (1000K). In this case the exothermal
heat obtained from the sulfidization is used for the heat of
dissociation required by the sulfur vapor (within the range
T2 ~ 1000K) and for realizing the other parts of the heat
balance. The temperature of the feed phases of the sulfidization
can thus be varied within a rather wide range, which is crucial
for control of the velocity of the sulfidization, the melting
of the complexes, and other factors.
The heating of the feed concentrate mentioned in Example 1,
usinq the heat of recombination (polymerization) yielded by
the sulfur vapor from the drying temperature of the concentrate
to the excitation point of the sulfidization reactions are
discussed below in order to illustrate the change in the
enthalpies in the control method.
The enthalpy of the feed concentrate corresponded to the
following Equation (01):
(01) ~He = 110.263 x 10 3T + 22.857 x 10 6T2 _ 26.820 Mcal/t
concentrate
The quantity corresponding to the example, i.e. 291.2 kg, is
taken as the quantity of elemental sulfur. The total enthalpy
of this quantity of sulfur (~He+f, Mcal) is in accordance with
Equation (02):
(02) log~He+f = -160.1033 + 184.5768 x 10 3T - 62.0020 x
10 6T + 46726.5/t
400K (127C) is taken as the temperature of the concentrate
fed into the sulfidizing system; this temperature corresponds
to the temperature of the product obtained from a conventional
drying cylinder. The elemental sulfur is vaporized to 1000K
(727C), at which t1-e the avera~e atomi- number of its vapor
. "' "' ' .
: ,
18 ~ Z5~
molecules is v = 2.17 and its enthalpy AHe+f'= 157.7 Mcal.
When the concentrate and the sulfur vapor meet each other, the
temperature and the enthal~ of the vapor decrease, while the
atomic number of the vapor molecules increases, and, respectively
the temperature and enthalpy of the concentrate increase under
the effect of the heat released by the polymerization of the
sulfur vapor. In this case the heat balance of the system
co~responds to the following Equation (03):
(03) 205.523 = 110.263 x 10 3T + 22.857 x 10 6T2 +
exp10 [ ~He+f / Sv]
The value obtained for the equilibrium temperature from Equation
(03) is T = 872K (599C). The average atomic size of the sulfur
vapor molecules increases from the initial value, v = 2.17,
to v = 4.18, while the volume of the qas decreases from 94 Nm3
to 49 Nm3, respectively.
Equation (04) is obtained or the thermal enthalpy of sulfur
vapor (~He~ heat of formation, not included), kcal/kg S:
(04) ~He = -161.567 + 470.813 x 10 3T - 214.667 x lQ 6T
The values in the following table are obtained for the change
in the enthalpy of sulfur vapor (Mcal/291.2) from Equations (02)
and (04):
H 1000K 872KDifferenceDifference, %
~He 27.54 24.972.57 3.93
~Hf 130.19 67.3162.88 96.07
He+flS7.13 92.2865.45 100.00
It'can be seen from the table that, when cool'ing from 1000K
to 872K, the sulfur vapor yields 62.88 Moal as recombination
energy for the heating of the concentrate from 400K to 872K.
The proportion of the thermal ~as enthalpy of the sulf~r vapor
of the total heat transfer of the ~as phase is only 3.93%
[~(~He) = 3.93 Mcal l. If the vaporization point of sulfur
(717.8K) is'taken as the reference state, the sulfur vapor
'.. ~, , , , , , 7 , .. . ..
: . , . : . . . ~ -
:
~ , . . .: . . . . . ,.: , . ".. .
:: : . : ''1' : .. : . .. ;~
h~Z~3
yields only 64.9~ of its recombination ener~y in the reaction
observed. It should be noted that the final temperature, 872K,
reached by the concentrate and the sulfur vapor is quite
sufficient for the ex.citation of the exothermal sulfidization
reactions. If, as the result of these reactions, the temperature
of the system tends to rise too much, the sulfur vapor begins
to re-dissociate, whereby the temperat-ure is buffered.
It can be observed from the calculations that the amounts of
heat produced by the changes in the atomic number of sulfur vapor
molecules are large, and on the other hand, the control of
these heat amounts is very easy, a fact which makes the control
method highly advantageous technologically.
Exam~le 2
~ ~ .
Example 2 illustrates the structural changes in a cobalt
concentrate corresponding to Example 1 and the formation and
vaporization of arsenic sulfides when part of the sulfur
required by the process is added to the system in the form of
pyrite.
The decomposition of pyrite corresponds to reaction (equilibrium
as in Example 1):
., ~
2--~ FelOS12 + (1/1-25 X)Sx(g)
The quantity of sulfur required for structural chan~es in the
concentrate and for the formation of the vaporization sulflde
is 182.12 kg S (851.80 k~ FeS2) per one tonne of concentrate.
The free elemental sulfur of the arsenic polymer is added to
the system by means of the feed gas phase, whereby the sulfur
pressure in the gas phase is that corresponding to Example 1,
i.e. PS = 0 50 atm. If only a proportion, Z, of the amount
of pyrite mentioned above is fed into the system, the additional
sulfur required is fed in a gaseous form ~P5 = 0.50 atm).
Example 2 with its balance of materials and heat balance is
illustrated in detail in Table 2.
: ~ -
. .
..
:~ ' - -: .
~3Z~3
It can be seen from Table 2 that, when the total sulfur quantity
required for the structural chan~es in the concentrate (65.24%
of the total sulfur) is fed in the form of pyritic sulfur, the
temperature of the feed ~as phase rises beyond technical control
(5616C). In reality this temperature is somewhat lower, since
part of the S2 molecules used in the calculations have dissociated
into Sl molecules, in which case the heat of polymerization
obtained is ~reater than that calculated (the pressure of
monoatomic sulfur vapor at 2500K is, however, only 10 5 atm).
When sulfur,vapor is fed ~to the syste~ at 1000K (727C), the quantity
of sulfur to be obtained from pyrite is only 45.95 kg. Howe~er,
this corresponds to only 15.8% of the feed of sulfur.
It can be concluded from these results that the use of pyritic
sulfur for producing structural changes in a process into which
heat is introduced only by pre-heating the concentrate and the
gas phase is of no si~nificance. It must be noted in particular
that introducing heat into the system with combustion gases
(i.e., the use of fossil fuel) lowers the partial pressure of
elemental sulfur so much that the process is not realized.
The result obtained is, of course, due to the act that the
decom~ositlon of pyrite is a highly endothermal process (and
also requires a high excitation tem~erature). The basic heats ~ -
of formation (~H29~) have been calculated for the following
table, using the heats of formation of the feeds and the
products (Mcal) per one tonne of the feed mixture according to
the casesdlscussed above:
Proportion SFeS2 feed product
Z kg Mcal Mcal Mcal
0 0 259.8 328.3 68.6
0.252 45.95 274.3 310.6 36.4
1.000 182.12 297.5 282.3 -15.1
It can be seen from the table that when only cobalt concentrate
(Example 1) is processed the structural change is strongly
exothermal. The addition of pyrite to the system makes thè
process endothermal.
-- 21
Z~
Thus, when an exothermal process is involved, the excess heat
of the system and a temporary rise in the temperature can easily
be prevented by a pyrite spray into the reaction chamber, since
pyrite rapidly binds the-excess heat by endothermal reactions.
~xcess heat is easily produced when the pre-heating temperature
of the feed concentrate has been raised above that required in
the process so as to enable the excitation of the exothermal
reaction when the pre-heating temperature rRquired by the
processing of the concentrate is far below the éxcitation point
of the sulfidization reactions of the concentrate. A conventional
product processed to a high level of sulfur concentration can
be used advantageously for lowering the temperature. In this
case this product discharges its excess sulfur endothermally
(the use of such a product also prevents the precipitation of
the reaction product with iron). For cooling, the temperature
of the sulfur vapor can, of course, also be lowered (use of
energy ~f dissociation), provided that this is possible
without causing the vapor to condense.
Example 3
The feed concentrate corresponding to Example 3 is the same as
that in Exam~le 1. However, the sulfidizin~ process is operated
within the stability range of cobalt-iron pyrite [(Co,Fe)S2].
The amount of cobalt present in the concentrate is sufficient
for raising the dissociation pressure of the Ca pyrite formed
to the range of the sulfidization temperature (1000 - 900K).
The heat of formation of the product concentrate (~H298, Mcal),
per one tonne of feed con`centrate, grows from the value
corresponding to Example 1, ~Hf = -302.~, to ~Hf = -3-32.6,
i.e. the increase in the exothermicity of the process is very
~reat (78.9 Mcal).
.
The great difference (113.5 Mcal/t) between the heats of
formation of the feed and product concentrates of the
sulfidization requires very low feed temperatures for both the
concentrate and ~as phases. In the reaction chamber of the
furnace the feed components can no longer react without
excitation. For this reason it is advantageous to feed a portion
of the elemental sulfur required for sulfidization along with
: . .
~ .3~
the concentrate, whereby the temperature and partial pressure
of the sulfur fed in gaseous state can be maintained at such high
level that, when the sulfur ~as cools in the reaction chamber to
the point oE excitation of the sulfidization of the concentrate,
it yields, while polymerizing, the heat required for the excita-
tion, whereafter the exothermal reactions realize the heat
balance of the process. Momentary rises in the processing
temperature can be rectified by intermittent dilution of the
sulfur gas phase of the system, whereby the dissociation of the
sulfur vapor and the heating of the dilution gas rapidly cause
a lowering in the temperature of the system.
The calculated balance of materials and heat balance
corresponding to Example 3 are given in Table 3.
It should be noted that the carrying out of the sulfidizing
process in a manner corresponding to Example 3, instead of that
corresponding to Example 1, increases the sulfur requirement,
but on the other hand it eliminates the unit for pre-heating
the concentrate from the process apparatus.
Example 4
In the case corresponding to Example 4, a nickel-antimonide
concentrate mineral (NiSb) is sulfidized so as to obtain pure
minerals as final products, i.e. sulfides of nickel and antimony.
The balance of materials and the heat balance illustrating the
various processing stages of the example are given in detail in
Table 4.
Nickel antimonide mineral is sulfidized usin~ the following steps
a) The nickel-antimonide mineral is sulfidized at approx. 1000K,
using a sulfur-vapor partial pressure of PS = 0 35 atm. The
temperature of the feed concentrate is 429K (156C) and that of
the feed gas phase, respectively, 750K (477C). On the basis
of the analyses performed, the reaction products obtained are
a solid nickel sulfide phase devoid of antimony and a molten
antimony sulfide phase devoid of nickel.
The heat of formation of the initial concentrate (~H298, Mcal/t)
is QH = -110.8 and that of the product obtained, respectively,
~H ~ -180.3 (i.e. ~H = -260.4 per 1.444 t of product. The
.
'
,, - . .
23 ~ z~ ~
exothermal heat released by the process is used for pre-heating
the concentrate (429--~ 1000K) and for heating the sulfidiz~ing
~as (750--~1000K). The sulfur vapor of the sulfidizing gas
phase dissociates, and its enthalpy (~He+f, kcal/kg) increases
from ~EIe+f = 208.4 to ~He+f = 549.4. The heat required for the
dissociation, ~(~He~f) = 341.0, is taken from the exothermal
sulfidizin~ reactions. In the sulfidization,the partial pressure
of the sulfur vapor in the ~as phase decreases ~PS = 0 35
PS = 0.10 atm). In this case, with the decrease in the partial
pressure, the unreacted part of the sulfur vapor dissociates
further so that at 1000K the heat of dissociation required is
17.9 kcal/k~ S.
b) The products of reaction and the gas phase pass from the
sulfidizing process to the cooling furnace. It can be observed
from the equilibrium dia~ram in Figure 5 that, when the product
cools down, the operation moves within the stability field to
the area of nickel pyrite (NiS2), while the antimony sulfide
remains liquid LSb2S3(1)]. The bulk of the sulfur contained in
the gas phase then becomes stacked on the surface of nickel
monosulfide so that when the ~as phase discharges from the
furnace its sulfur content is only about one percent by volume
(890K). The antimony sulfide present in the gas phase becomes
condensed in the reaction chamber of the coolin~ furnace. The
thermal losses of the coolin~ furnace are obtained from the
increase in the exothermicity of the reaction ~roducts (NiS 7
NiSX ~ NiS2, ~He+f = -29.8 Mcal/1.444 t of feed mixture) and
from the change in the heat content of the gas phase (cooling:
1000K-~890K). It should be noted that the quantity of the
final reaction products obtained from one tonne of antimonide
concentrate i3 1 . 545 tonnes. The respective increase in the
exothermicity of the reaction products is 179.4 Mcal/t feed
concentrate.
By regulating the amount of sulfur in the sulfidization product
in the cooling furnace it can be determined whether the antiomny
sulfide is at the bottom of the tank or on the surface in the
cooled product, or whether or not the sulfides of antimony and
nickel are mechanically mixed with each other. Since the
~ .
,, . " . . . ............ . . . .
:
:' ' : , : . ~:: : ::
24
~Lh~ ~Z~r~3
concentrates also often contain iron, this control is simple.
c. From the cooling furnace the sulfur- and antimony-low
gas phase is fed to the feeding end of the sulfidizing apparatus,
where elemental sulfur arrives from the sulfur vaporizer.
When sulfur is vaporized at 700K, its heat content is ~He+f =
170.4 kcal/kg Sv (v = 6.97). The temperature of the sulfur
fed i~to the sulfidizing process is 750K and its heat content
is ~He+f = 208.~ kcal/kg S. The thermal energy required for the
dissociation of the sulfur, 3-8.1 kcal/kg S, is yielded by the
gas phase obtained from the cooling furnace.
E~ample 4 illustrates clearly the versatile ways in which the
dissociation-recombination energy of sulfur vapor can be
utilized in sulfidizing processes.
Example 5
The behavior during sulfidization in order to obtain structural
changes, of technologically highly important co~plex minerals
containing a high amount of impurities and belonging to the
Cu-As-S basic system is reported below in greater detail than
the behavior of the minerals discussed above.
,
The fahlerz series, of the general form (Cu, Ag)l2(Cu,A~, Fe, ;~
Hg, Ge,Sn)l2(As, Sb, Bi)8S24S2, which is structurally analogous
to zinc sulfide of the sphalerite type (ei~ht-fold elementary
cell volume), and the enargite series, of the general form Cu3
(As, Sb)S4, which is analogous to zinc sulfide of the wurzite
type, are discussed below as a technologically important
group of minerals.
' ' , ,
With some exceptions, these groups of hydrothermal minerals
usually appear in the same ore deposits.
Tennantite (Cu24As8S26) of the fahlerz series and enargite
(Cu6AsS4) of the enar~ite series a~pear as compounds of the
; basic system Cu-As-S. This system also includes sinnerite
(Cu6As4Sg) and lautite (CuAsS), both ra~e ih nature, as well
p (Cu24Asl2S31), which participates in the phase
reactions but is unknown in nature. The sulfidization of the
...... . . . .
~ ` ` , ' : '
,
,
~ .3Z~`B
compounds mentioned above is encumbered by the low melting ranges
of the compounds and the phase r~actions between the same.
For this reason, special sulfidizing conditions are usually
required.
The following are some examples of the melting conditions of
the compounds and the reactions:
- sinnerite melts incongruently to liquid and compound A at
489C
- enargite reacts with compoul~ A to tennantite and iquid at
573C, and compound A decomposes to tennantite and liquid at
578C
- tennantitè having the composition Cul2 31As4S13 melts at 665 C
- enargite for its part decomposes to an enargite about 2% atomic ~ -
As-poorer (~Cu3Aso 84S4) and liquid at 666C. The maximum
melting temperature of enargite compositions is not known.
The Cu-As-S system has never been studied under a controlled
sulfur pressure, and so the melt phase ranges and melting
points corresponding to the above and any other reactions may
deviate substantially from the values measured at low total
pressure. In sulfidizing experiments it has been observed that -
tennantite can be converted to enargite at temperatures >700C,
and the enargite, for its part, when the arsenic vaporizes as
a sulfur polymer, can be converted to digenite and/or chalco-
pyrite and kernite, depending on the iron amount present or
added. Sintering of the concentrate or visible appearance
of melt phases cannot be observed during treatment. The
equilibrium constants (numbered) relating to the sulfidizing
of tennantite-enargite systems, calculated according to the
avallable thermodynamic values (imprecise~, correspond to those
given in the table on the following page. ~he stability ranges
of the compounds most important considering this discussion are
indicated in the equilibrium diagram in Figure 5 as a function
of the sulfur pressure and the temperature.
` .` ' '
..,
~ . ' ' .
.. .: : . ... :
: - , ::: .. ~ ,
- :: . - . .
~$~3Z5~3
~r '`D~ O N Ln 1` ~ O
CO ~ n ~r ~ o
~9 ~ Ln00 ~ o
Ln co ~r
r~ ~ ~ Ln
~ o co ~r r
00 ~ ~D ~ ~J ~ N
O Ln ~ OD O
m I , ~ ~ o ~
~ a~ co ,~ ~ ~ Ln
O ~ O U:~N~r O r~
er Ln ~ O ,~ ~ ~D O
o ~ Ln ~ ~
o ~O ~ ~ ~ oO
Ln1~ ~ ~I N
~_) I . I . .
V~ t ~ N ~r Ln OD ~ 0
~ r oo Ln ~r Ln
tJ~ ~ ~ OD ~ ~ 1-- Ln Ln O Ln
O ' ~ + ~ +
m
+
tn ~
. . _
P ~ U~
K --
o
_ D +
U~ +
U~ ~1 ~ _ + +
+ ~ ~
~ a) -- ~ _
~ ~ ~o U~
U U~
;r + ~ +
Q ~ 3 + V~
~ +U~ ~
oK _ ~ _ 3 + +
o S
o _ LD ~ Q)
I` + S ~
o ~ U~ ~
~ : ~ +
+ + + ~ ~
h ~ ~ U ~1 ~ ~1
~rl
.
æ ~1 ~ ~ ~ Ln ~ ,~
ZSB
Reactions 1-9 in the table:
It can be observed from the extrapolated equilibrium values of
Reaction 1 that enargite becomes stable at 1000K at a sulfur
pressure above PS = 3 49 x 10 2 atm.
According to Reaction 2, the enargite dissociates to digenite
and ~aseous arsenic sulfide. At a sulfidization temperature of
1000K the free energy of Reaction 2 is positive. When a high
partial pressure of sulfur is used, a digenite phase (Cu2 ~S),
short of the full capacity of the metal,is formed in place of
chalcocite (Cu2S), whereby the activity of chalcocite decreases
to one advantageous to the equilibrium reaction. The equilibrium
constant of the reaction
)Cu2s(ss) + (~/2)S(ss) = Cu2 ~S
and the activities of sulfur and chalcocite can be calculated `
from known measured values. Within the temperature ranqe 823-
1023K (550-750C) and with ~ values of ~ = 0.14-0.24, both the
~ and aCU S functions are of the form
aCu2S 2~ + 1.3222 - 2C3.7/T
(~/4/log PS = 0 3335 ~ 460.0/T - (1 - ~)log aCU S
When the sulfur pressure corresponding to the tennantite-
enargite equilibrium, PS = 3-49 x 10 2 (1000K), is used, the
equilibrium digenite obtained from the equations corresponds
to the composition Cul 86S and the chalcocite activity obtained
i5 acu S = - 81. In this case the vapor pressure of arsenic
PAs4S6 0.56 atm so that Reaction (2) proceeds
advantageously. Owing to the positive nature of the free
energy of the reaction, the vapor pressure of arsenic sulfide
dëcreases when the partial pressure of the vapor of sulfidizing
sulfur is raised. On the other hand, the lowering of the partial
pressure of sulfur results in that the arsenic is not removed
from the system, since when the sulfur amount in the digenite ?
decreases the solid solubility of arsenic in it increases
(e.g. 500 C, composition (Cu, As)l 92S, where As2O3 ~0.70% by
.. ~ - . .- . . , : .
, : : . - . : . ~:
28 ~ zSB
wei~ht). It is of particular interest that melt phases which
encumber the process are also produced if the sulfur amount
in digenite decreases, i.e. if the partial pressure o~ sulfur
in the system is lowered. The same observations apply to
Reaction 7 as to Reaction 2. However, it should also be noted
that in the tennantite-digenite equilibrium the solid solubility
of digenite and arsenic is even greater than under the conditions
of Reaction 2. In performing the vaporization sulfidization,
sulfur pressures above those of the tennantite-enargite conver-
sion are thus effective.
.
Each of Reactions 3, 4, 5, and 6 is realized. When iron
sulfide or iron is added to enargite, the products of the
reaction are bornite and/or chalcopyrite. It can, however, be
observed in carrying out the sulfidization that Reaction 3
proceeds poorly. Results are not obtained from Reactions 4,
5, and 6 at the computed limit pressure of sulfur, since under
these conditions the sulfide of arsenic is not stable. It
should also be noted that, when low sulfur pressures are used,
the reactions proceed slowly and there is a risk of the
appearance of stationary melt phases. The above observations
apply to tennantite~chalcopyrite reactions according to Reactions
8 and 9.
:
Table 5 shows the calculated balance of materials and heat
balance for sulfidizing processes of conventional iron-bearing
enargite concentrate (~ by weight: 30.5 Cu, 11.8 As, 0.3 Sb,
35.7 S, and 15.8 Fe), in which both the arsenic and the antimony
vaporize as sulfides and the original enargite matrix is
rearranged to correspond to the chalcopyrite-bornite-(digenite)
equilibrium.
Values easy to achieve technically have been placed in the
general outcome of the balances for the partial pressure of
sulfur vapor and for the arsenic concentration in the arsenic-
sulfur polymer, i.e. PS = X = 0.8 atm and Y = 0~4 (40% by
weight As). In the case of the concentrate under discussion,
the sulfidizing process is endothermal, and therefore additional
heat is introduced into the system by burning part of the sulfur
. ,~ .
.
29
L3Z5~3 :
~eed. Owing to the endothermal character of the process, the
concentrate is fed into the system at a high degree of
pre-heating, i.e. in this case at 500C. The significance of
a high sulfur potential for the quantitative and kinetic removal
of arsenic and for the elimination of the formation of melt
phases has already been discussed above (the solid solubility
of arsenic in the chalcopyrite-enargite equili~rium is equal to
or ~reater than in a corresponding digenite equilibrium).
In the technical implementation of the sulfudizing process,
the high sulfur pressure is of crucial importance, especially
as regards endothermal sulfidizing reactions. This point is
discussed briefly in the summary below:
a) It can be observed by a differential-thermal analysis that '~-
enargite concentrate is excited in a sulfur atmosphere to
produce a reaction sufficiently rapid technically at 570C
(the speed at which the temperature increased was 6C min 1 and
the corresponding increasing negative temp,erature gradient of
the endothermal reaction was -1.4C min 1).
.
b) The entha,lpy of the enargite concentrate (Mcal/t)' was
~He = 145.989 x 10 3T - 39.323, and the enthalpy including the
heat of formation, respectively, QHe+f = 145.989 x 10 T -
219.898. The total enthalpy of the products of sulfidization
(product sulfide + arsenic sulfide) was~He+f = 194.570 -
10 3T - 24,3.709.
c) The values obtained for the enthalpy of the vapor (~He~f, '-
kcal/kg S) from the temperature functions of the total enthalpy
of sulfur vapor at temperatures 1100, 1000, anq 845K are
575.1, 541.7, and 276.1, respectively. ~t the same temperatures
the total enthalpies of vapor corresponding to a pressure of
P' = 0.i atm are 582.6, 567.3, and 492.1, respectively.
Sx
According to the balance of materials in Table 5~, the sulfur feed
requirement'of the process per one tonne of concentrate is 33.73
kg and this'plus the sulfur quantity required for providing
additional heat total 95.49 kg. The heat yielded into the system
'
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by the polymerization of the sulfur vapor, when the temperature
of the vapor decreases from the feed temperatures 1000K and
1100K to the excitation point of the concentrate, is in
accordance with the following table, calculated from the values
given:
PS ~ atm ~T, K ~He+f~ Mcal
33.73 kg S 95.49 kg S
0.8-1.0 1100-845 10.09 28.55
1000-845 8.96 25.36
0.1 1100-845 3.05 8.64
1000-845 2.54 7.17
It can be seen from the table that,when the operation is
carried out at a high partial pressure of sulfur, the heat of
polymerization of the pre-heated sulfur vapor provides the heat
required for heating the concentrate from the pre-heating
temperature, 773K (500C), to the excitation temperature of
the reactio~s, 845K (572C). According to point b) this heat
is ~EIe = 10.15 Mcal. Thus a large amount of the heat of poly-
merization remains unused. When the operation is carried out at
a low pressure, the heat of polymerization is not sufficient
even for heating the concentrate.
d) Within the range of the excitation temperature of the feed
concentrate, arsenic-antimony sulfide is formed as a molten
pure phase which separates from the matrix. When using the
total enthalpy values given in point b), the value obtained for
the enthalpy of the concentrate a~ 7?3K is ~He+f = -107.05 and
that obtained for the enthalpy of-the products at 845C is
~He+f = -79.30. The heat required for both the heating of the
concentrate and for satisfying the heat requirement of the
endothermal reactions ~excluding the heat losses of the system)
i9 thus 27.75 Mcal. It can be observed from the table in point
c) that the heat yielded by the polymerization of sulfur with
a high partial pressure is approximately sufficient for realizin~
the sulfidization at a low temperature. When the operation is
carried out at a low partial pressure of sulfur, the releasing
o~ the polymerization energy of the vapor does not fall within
.
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the range applicable in the process, but within a temperature
range approx. 100K lower.
e) The (As,Sb)2S3(1) phase formed within the excitation
temperature range o sulfidization must be vaporized in the
process. The temperature of the system must therefore be raised
in order to reach the vaporiza-tion temperature. The heat required
both for covering the thermal losses of the furnace and for the
vaporization enthalpy is appropriately obtained for the process
by burning part of the sulfur vapor.
The utili~ation of the heat of polymerization of sulfur vapor
in the manner described above also makes it possible to burn
the sulfur vapor only after the sulfidization reactions have
started, whereby the ore matrix which is now at least partly
rearranged tolerates (without sintering or melting) the
temperature of the flame and the combustion gases.
Some cases (A, B, C, and D) of various sulfidizing methods have
been calculated for the sake of comparison in addition to the
general case in Table 5.
In cases A and Al the enar~ite concentrate is sulfidized
conventionally with pure sulfur vapor. In case A the feed
temperature of the sulfur vapor is 900K, and in case Al it is
1000K (i.e. the same as the temperature of the products).
In case A the change in the total enthalpy of the gas phase
from the feed temperature to the product temperature is
~He+f = 163.05 kcal/kg, of which the proportion of gas enthalpy
He) is only 4.61%. The dissociation energy required for
raising the temperature of the sulfur (95.99 kg) of the gas pha~e
within the temperature range 900 - 1000K is thus
~Hf = 14.85 Mcal. In case Al this amount of energy is not
required and so the amount of sulfur used for burning is
respectively (15.09 Mcal) less than in case A.
.
In cases B and Bl, iron powder has been added to a concentrate
corresponding to the previous case in such a quantity that
.. . . . , ~ -
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~.3Z~3
all of the sulfide obtained as a product is chalcopyrite. It
can be observed from the corresponding balance of materials
and heat balance that the consumption of sulfur has increased
sharply in comparison with the previous case. Out of the sulfur
vapor fed at a temperature of 1000K, however, only 15.26 kg
is burned for the heat required by the endothermal reactions,
this amount being 3.6 times smaller than in case A. This is,
of course, due to the increased endothermicity of the system
(the iron is sulfidized). In case C, the additional heat
required by case B has been introdu~ed into the system by
burning iron corresponding to the equilibrium FeSl 27. The
amounts fed were 49.99 kg Fe and 36.45 kg S. The outlet gases
are thereby obtained devoid of sulfur dioxide.
In case D the iron required for obtaining the chalcopyrite of
the final product was added to the system as pyrrhotite (FeSx).
In this case the amount of sulfur to be burned in order to
produce the heat required for the endothermal reactions (and
heat losses) has increased from the value corresponding to
case B, 15.26 kg, to 44.37 kg. This is due to the increased
endothermicity of the process, since the exothermal heat of the
sulfidization of the iron phase is complete~ly absent.
The sulfidization of pure enargite mineral to the bornite
stage under conditions corresponding to the above examples
(X = 0.8; Y = 0.4) is discussed briefly below:
Corresponding to reaction
5Cu3AsS4 + 3Fe ~ 3Cu5FeS4 + 5/4As4S6(g) + 1/4S2
an amount of 85.09 kg of iron and 180.92 kg of sulfur must
be fed per one tonne of enargite. The sulfur released according
to the reaction amounts to 8.14 kg. Out of the total sulfur
feed, 25.82 kg of sulfur must be burned in order to produce
additional heat~
: .
When the iron is replaced with pyrrhotite, the amount of sulfur
fed is 154.55 kg, QUt of which 48.30 kg of sulfur must be
"! burned in order to realize the heat balance. The quantity of
.
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sulfur released in the reactions i5 57.00 kg S. When the
required iron is added in the form of pyrite, the sulfur feed
amounts to 126.53 k~, out of which the share of sulfur to be
burned is 69.13 kg. In this case the amount of sulfur released
in the reactions is 105.85 kg.
When the additives are fed into the system at 773K and the
products [Cu5FeS4ts) + As4S6(~)] are withheld from the system
at 1000K, the followin~ values are obtained for the endo-
thermicity of the reactions per one tonne of enargite when
using various additives: 29.99/Fe, 59.68/FeS, and 85.25 Mcal/
FeS2. At 1000K the heats of formation of the product compounds ~-
and enargite (Mcal/t) are as follows: -73.06/Cu5FeS4,
-117.74/CuFeS2, and -27.40/Cu3AsS4.
I~ can be observed from the examples that the sulfidization of
enargite or its concentrates to chalcopyrite or bornite by
adding the lackin~ amount of iron in the form of iron or its
suIfides is usually not advanta~eous, since the amounts of sulfur
feed easily increase with increased endothermicity of the
reactions (in spite of the fact that the final s~lfide products
are more stable than the inltial products).
To summarize, the utilization of the heat of dissociation-
recombination of sulfur vapor is crucial in the treatment of
the endothermal process under discuss~ion. It is self-evident
that, when the mineral composition of the ore chan~es, the
conditions, temperatures and partial pressures of the sulfidizing ;~
process must also be changed according to need.
Example 6
This example illustrates the chlorination of molten copper
.
sulfide matte in order to va?orize, in the form of chlorides,
the impurities present in it. The molten matte o the example
corresponds to the so-called converter matte, from whi¢h the
iron has already been sla~ged.
;~
` The converter matte is in equilibrium with metallic copper
(aCU ~0.99), and it usually already contains a large amount
of metallic copper, both dissolved without char~e (Cu) or
34
in inequillbrium. It can be assumed -that a large amount of
impurities adhere to this copper. Accordin~ to measurements, the
dlstribution of impurity components as regards copper sulfide
and the copper in equilibrium with it at 1250C is as follows
(% Me in Cu2S/~ Me in Cu): 0.57/Zn, 0.12/Pb, 0.10/Sn,
0.06/Sb, and 0.15/si. When an attempt is made to chlorinate the
melt as such, the copper chlorinates well and the impurities
chlorinate poorly. When elemental sulfur is added to such a
melt before chlorination or during it, the melt takes it in
in excess when compared with the stoichiometric chalcocite
(Cu2S), e.g. in the case under discussion: PS = 0.8 atm,
CuxS:X = 1.902. Thereby the copper activity of the copper in
the sulfide melt decreases taCU ~10 ), and the impurities can
be chlorinated selectively.
The impurity chlorides formed during the chlorination are so
stable that in spite of heat-binding vaporization the total
process is strongly exothermal and the temperature of the melt
usually rises drastically. Using the large amount of energy
required by the dissociation of sulfur vapor molecules, the
speed and amount of the increase in the temperature of the melt
can be buffered to the desired values, by feeding in the
gaseous sulfur at a minimal temperature. The excess of sulfur
which perhaps ~is used can be recovered and returned to the
process when condensing the chlorides.
Table 6 shows in detail the above chlorination process for
converter matte.
' -' .
When the converter matte of the example is maintained unchanged
during the chlorination process (1500K), the heat bound by the
sulfur vapor is 24. a Mcal (700K-~1500K). When the melt
temperature is allowed to increase by 50K, the heat bound by
the dissociation is I7.8 Mcal. These amounts of heat are very
great, for they represent, respectively, 33.2% and 25.3% of
the potential total heat of the heat balance.
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