Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1077~
Ihis invention relates to methods of desulphurizing
fluid materials and particularly to a method of external de-
sulphurizing fluids such as molten iron and steel, stack gases,
coal gases, coal liquification products, and the like using
rare earth fluorocarbonates or rare earth oxyfluorides in an
essentially dry process.
According to the invention, there is provided a
method of desulphurizing fluid materials comprising the steps
of:
(a) reacting a member from the group consisting of
rare earth fluorocarbonates and rare earth oxyfluorides at low
oxygen potential with sulphur to be removed from the fluid
material to form one of the groups consisting of rare earth
sulphides and rare earth oxysulphides and mixtures thereof, and
(b) removing said oxysulphides and sulphides.
As we have indicated above this method is adapted to
the desulphurization of essentially any fluid material. We
shall, however, discuss the method in connection with the two
most pressing problems of desulphurization which industry
presently faces, i.e. the desulphurization of molten iron and
steel baths and the desulphurization of stack gases.
External desulphurization of molten iron and steel
has been practiced for quite some time. It is a recognized,
even necessary practice, in much of the iron and steel produced
today. In current practices for desulphurization of iron and
steel it is common to add magnesium metal, mag-coke, calcium
oxide, calcium carbide or mixtures of calcium oxide and calcium
carbide as the desulphurizing agent. Unfortunately, there are
serious problems, as well as major cost items involved, in the
use of all of these materials for desulphurization. Obviously,
both CaO and CaC2 must be stored under dry conditions, since
- 1 -
D ~
76~Z
CaO will hydrate and CaC2 will liberate acetylene on contact
with moisture. Magnesium is, of course, highly incendiary and
must be carefully stored and handled. There are also further
problems associated with the disposal of spent desulphurization
slags containing unreacted CaC2.
We have found that these storage, material handling
and disposal problems are markedly reduced by using rare earth
fluorocarbonates or oxyfluorides in a low oxygen content bath
of molten iron or steel. The process is adapted to the desul-
phurization of pig iron or steel where carbon monoxide, evolvedby the reaction, where carbon is used as a deoxidizer, is di-
luted with an inert gas such is used as a deoxidizer, is diluted
with an inert gas such as nitrogen or by vacuum degassing the
melt in order to reduce the oxygen potential and thereby increase
the efficiency of the reaction by reducing the likelihood of
forming oxysulphides. The principle may also be used for de-
sulphurizing stack gases from boilers, etc., as we shall discuss
in more detail hereafter.
In desulphurizing molten iron and steel in the
practice of this invention we preferably follow the steps of
reacting rare earth oxyfluorides, rare earth fluorocarbonates
and mixtures thereof including bastnasite concentrates in the
presence of a deoxidizing agent with the sulphur to be removed
to form one of the groups consisting of rare earth sulphide and
rare earth oxysulphide and mixtures thereof.
Preferably, hot metal is treated in a ladle or trans-
fer car with rare earth oxyfluorides or fluorocarbonates, by
the simple addition and mixing of the rare earth oxides, by an
injection technique in which the rare earth compounds are in-
jected into the molten bath in a carrier gas such as argon ornitrogen or by the use of an "active lining" i.e., a rare earth
- 2 -
107768Z
compound lining in the vessel.
Similarly, the problem of desulphurizing gases is one
of the oldest recognized problems in environmental chemistry.
It dates back to the beginning of the utilization of fossil fuels
for home heating and for industrial power. Sulphur dioxide is
the primary sulphur compound which has been recognized as the
problem in environmental control. Sulphur dioxide is a constit-
uent in many waste gases such as flue gases, off gases from
various chemical manufacturing processes, stack gases from coal
and oil burning furnaces and boilers, smelter gases, ore roaster
gases, coke gases and the like. Contamination of the atmosphere
by sulphur dioxide, whether present in dilute concentrations of
0.05 to 0.3 volume percent as in power plant flue gases or in
higher amounts of up to 10% as in ore roaster gases, has been a
public health and environmental problem for many years due to
its effect on the respiratory system of animals and humans, its
destructive effect on plant life and its corrosive attack on
metals, fabrics and building materials.
The reduction or elimination of the sulphur dioxide
from gases emitted into the atmosphere is an essential key to
the successful use of the world's abundant fuels (coal and high
sulphur oils). Thus many methods have been proposed for the de-
sulphurization of gases. Most methods which have been proposed
are technically feasible but their expense is in most cases
; completely prohibitive. The most commonly proposed methods in-
volve scrubbing the gases with water and precipitation of the
sulphur dioxide with lime as calcium sulphate or sulphite, de-
pending upon the process involved. Unfortunately, the expense
of scrubbing the vast amounts of gas involved and disposing of
the resulting precipitate is extremely expensive.
The present invention lies in the discovery that rare
~ - 3 -
. ?~ '``
' 107768Z
earth fluorocarbonates and rare earth oxyfluorides will, at lowoxygen potential, remove sulphur from gases and will in turn
give up the sulphides at high oxygen potential so that they may
be regenerated with the production of a gas high in sulphur
oxides from which elemental sulfur, sulfuric acid and like
useful prod~cts can be obtained.
rhe ability of ~astnasite
- 3a - :
. . .
- . ..... - - ~ . :
.. : . . . . . . : - . .
1o776b?z
concentrates (complex rare earth fluorocarbonates and rare
earth oxyfluorides) to transform to oxysulphide and sulphide
under conditions of low oxygen potential has been established
thermodynamically and experimentally by applicants.
In all cases the chemical reactions are as follows:
I. Desulphurization at low oxygen potential,
2CeO2(s) = Ce2o3(s) + l/2 2(g~ ~-
2 3(s) l/2 S2(g) = RE2O2S( )+l/2 O
RE O S + S = RE S +O
2 2 (s) 2(g) 2 3 2(g)
II. Regeneration at high oxygen potential,
2 3(s) 3 2(g) = RE2O2S( )+ 2 SO2( )
2 2 (s) / 2(g) RE2O3(s)+so2(g)
e23(s) + l/2 2(g) = CeO2(s)
In the case of liquid reactors, such as molten iron
or steel, the product sulphide or oxysulphide will either be
fixed in an 'active' lining or removed by flotation and absorbed
into the slag cover and vessel lining depending upon the process
used for introducing the rare earth oxide.
The products of desulphurization of carbon saturated
iron with RE oxides is dependent on the partial pressure of
CO, pCO, and the Henrian sulphur activity in the metal, hs~
Using cerium as the representative rare earth, the following
standard free energy changes the equilibrium constants at
1500C for different desulphurization reactions can be
calculated from thermodynamic data in the literature:
~o776s2
- - - ---
REACTION ~ G cal. K1773
...
2CeO2(s)+[c] = ce2o3(s) C (g) 66000-53,16T pCO = 3041
Ce2O3(s~+[C]+[s]lw/O Ce22s(s) C (g) 18220-26,43T pCO~hs=3395
Ce22S(S)+2[C]+2[S]lW/o=Ce2S3(5)+2CO(g) 66180-39~86T P / S
3/2 Ce2o2s(s)+3[c]+5/2[s]l = Ce S4 +3CO( 127050-72.lT p3CO/h 5/2=1.25
Ce22S(S)+2[C]+[S]lW/o = 2CeS(s)+ 2CO(120,860-61.0l p2CO/hs=.027
(s) / 2(g) CO(g)-28200-20.16T pCO/p O2=7.6x10
_ -31520~5.27T hS/pl/ S2=5.4x10
The thermodynamics of desulphurization with lanthan-
ium oxide, La2O3, are similar although, in this case, LaO2 is
unstable and there will be no conversion corresponding to
CeO2~ Ce2O3.
In the case of desulphurization of gases, such as
stack gases, assuming the following gas composition at 1000C:
Component Vol.
C2 16
CO 40
2 40
2 4
H2S 0.3 3
(200 grains/100 ft .)
This equilibrium gas composition is represented by point A
on the diagram illustrated as Figure 6 where CO/CO2 = 2.5
and H2/H2S = 133. This point lies within the Ce2O2S phase
field and at constant CO/CO2 desulphurization with Ce2O3 will
take place up to point B. At point B, H2/H2S -104 and the
5.
-- . . . : . .
. .: :: . : : : : ,. .
''~ , . ' :: ' ' : . . : '
1~77682
concentration of H S is 0.004 vol.% (~3 grains/100 ft. ).
Beyond this point, desulphurization is not possible.
The basic theory for this invention is supported
by the standard free energies of rare earth compounds likely
to be involved. Examples of these appear in Table I which
follows:
w~ o
~ ~ u~ WU~ ~U ~n w w ~
11 11 11 11 11 11 11 11 11
~ ~ _ ~ ~ ~
~ ~ ~ ~ u~ ~ w w ~, ~
U~ ~ ~D
~u~ ~ ~ ~o ~o
~ ~ ~D'
~ O
~ . ,
O ~~ ~ W ~ ~ ~ ~
`I O W 1-- W ~ ~0 Ul ~ ~
O W (.rl C~ CO CO Ul N O X O
* ~ ~
~ 9 ~C ~
O o w o ~ ~ o o ~n
*-*
WI~-~WI~ IWI
O O O O O O O O O --O
g g o g g og g g g 7 D
I+ I+ I+ I+ I+ I+ I+ I+ I+ ~ ~ ~)
~ 0- 10- ~ w W W 0~ ~
~ . . . ,_ ~ ~ '
~h ' .
, '
6.
: . - ., :
lv7768z
The three phase equilibria at 1273K for the
Ce-O-S System is set out in Table II as follows:
. . . . _ . _
~ ~ ~ 2 2 2 2 2
W~ ~
_ _
æ
.q~ 2N ~æ
~ _ tn . .':
+
~ ~ .
(D
I I I 00 CO ~ h3 ~ ~-,
a~ n ~ co Q ~ T ~
~ w o ~ ~ o 1~
. ,~ _ W
.; ~ n _ ~
,~,o~ X X X 1- o 11 ~1
1l !1 1l ~ '- W X o x
x ~ x ~ ~ l
--- ~ -
:: -
~07768Z
Typical calculations of energy changes involved inthe systems involved in this invention are as follows:
2(~ + Ce22s(s~ = Ce2S3( ) + O
Ce2S3(s) = 2ce(~) + 3/2 S2( ) ~G = 351160 - 76-0T cal-
Ce2o2s(s) = 2Ce(Q) + 2(g) + 1/2 S2( ) : ~ G = 410730 - 65 OT cal
Ce O S( ) + S2(g) = ce2s3(s) + 2(g)
~ 1273K aG = 73573 cal. and pO2/pS2 = 2.33 x 10
';
. . . _. _ ~
23(s) + 1/2 S2(g) = Ce202S + 1/2 2(
Ce23( ) = 2ce(~) + 3/202( ) ~ G = 425621 - 66-OT cal-
10~ Ce22s(s) = 2Ce(~) + 2(g) + 1/2 S2(g) ~ G = 41n730 - 65-0T cal-
O + 1/2 S = Ce202s(s) + 1/2 2(g)
~1273K ~G = 13618 cal. and (pO2,~pS2) / = 4.6 x 10
:
Ce22s(s) + 1/2 S2(g) = 2CeS(s) + 2( )
Ce22s(s) = 2ce(~) + 1/2 S2(g) + 2(g) ~ G = 410730 - 65-0T cal-
2Ces(s) = 2Ce(~) + S2( ) : ~ G = 264960 - 49.8T cal.
~077682
Ce O S( ) + 1/2 S2( ) = 2CeS(s) 2(g)
1273K ~ G = 126420 cal. and po2/pl/ S2= 1.96 x 10
2 2_(s) + 5/2 S2(q) = 2ce3s4( ) + 3 0
2Ce3S4( ) = 6Ce(~) + 4S2( ) : a G = 966360 - 196.4T cal.
3ce2O2S(S) = 6Cet~)+ 3 2(g)+ 3/2 S2( ):L~G = 1232190 - 195.0T cal.
.
2 2 (s) / S2(g) 2Ce3S4(s)+ 3 2(g) ~G = 265830 + 1.4T caL
1273K ~ G = 267612 cal and p 02/P5/ S2 = 1.12 x 10
Ce3S4( ) = 3CeS(s) + 1/2 S2(g)
ce3s4( ) = 3Ce(~) + 2S3( ) : ~ G = 48318 - 98.2T cal.
10 3CeS(s) = 3Ce(D) + 3/2 S2( ) : ~ G = 397,440 - 74-7T cal-
. ~ . .
Ce3s4(s) = 3ces(s) + 1/2 S2( ) ~ G = 85740 - 23-5T cal-
1273X ~G = 55824 cal pl/ S2 = 2.6 x 10
~ '
2 3(s) 2ce3s4( ) + 1/2 S2(
2Ce3S4( = 6Ce(~) + 4 S2( ) : ~ G = 966360 - 196.4T cal.
~077682
3ce2S3( ) = 6Ce(~) + 9/2 S2( ) : ~G = 1053480 - 228.0T cal.
.
2 3(s) 3 4(s) + 1/2 S2( ) ~ G = 87120
@ 1273K A G = 468893 cal. and p / S2 = 8.9 x 10 9
2(g) 2(g) 2 (g)
. _ .
H2( ~ + 1/2 S2(g) = H2S( ) : ~G =-21580 + 11.80T cal.
@ 1273K ~G =-6559 and pH2S/(pH2.p / S2) = 13-4
pH2/pH2S log PS2
1 - 2.25
1o2 - 6.25
104 -10.25
6 -14.25
8 -18.25
-22.25
2 -26.25
(g) 2(g) 2 (g)
H2( ) + 1/2 2( ) = H2O : ~ G = -58900 + 13.1T cal.
@ 1273K ~G =-42223 cal. and (pH2/pH2O) pl/202 = 5.6 x 10 8
10 .
t 077682
pH2/pH2O log PO2
10 4 - 6.5
~2 -10.5
1 -14.5
2 ~18.5
104 -22.5
6 -26.5
8 -30.5
(g) / 2(g) CO2(g)
CO(g) + 1/2 2( ) = C2(g) : ~ G = -67500 + 20.75T. cal.
1273K a G = -41085 and pC02/(pCO.pl/202) = 1.1 x 107
PCO/pCO2 log PO2
- 6.1
10-2 -10.1 ~ - ',,'
1 -14.1
2 -18.1
104 -20.1
6 -24.1
lo8 -30.1
In the foregoing general description of this invention,
certain objects, purposes and advantages have been outlined.
Other objects, purposes and advantages of this invention will
be apparent, however, from the following description and the
accompanying drawings in which~
~` , . . .
107768Z
Figure 1 is a stability diagram showing w/o sulphur
as partial pressure of CO;
Figure 2a and 2b show Ce2S3 and Ce2O2S layers on a
pellet of CeO2;
Figure 3 is a graph of the theoretical CeO2 required
for removal of 0.01 w/o S/THM;
Figure 4 is a graph showing the volume of nitrogen
required to produce a given partial pressure of CO;
Figure 5 is a graph showing the CeO2 requirements as
a function of partial pressure of CO; and
Figure 6 is a stability diagram for stack gas systems
treated according to this invention.
Referring back to the discussion of free energy set
out above, it is clear that these free energy changes may be
used to determine the fields of stability of Ce2O3, Ce2O2S, : -
Ce2S3, Ce3S4 and CeS in terms of the partial pressure of CO and
the Henrian sulphur activity of the melt at 1500C. The
resultant stability diagram is shown in Figure 1, the boundaries
between the phase fields being given by the following relation-
ships:
. --~
BOUNDARY EQUATION
Ce23 ~ Ce O S log pCO = log hS + 3.53
Ce O S - Ce S log pCO = log hS + 0.28
Ce2O2S - ce3s4 log pCO = 0.83 log hS + 0 03
Ce2O2S CeS log pCO = 0.5 log hS ~ 0 79
Ce2S3 - Ce3S4 log hS = ~ 1.47
_ log hS = ~ 2.45
12.
~o~6~Z
The phase fields in Figure 1 are also shown in terms of the
Henrian activity of oxygen, ho~ and the approximate [w/o S]
in the iron melt using an activity coefficient fs~ 5.5 for
graphite saturated conditions.
The coordinates of the points B, C, D and E on the
diagram are given below:
. _
COORDINATES B C D E
~ ._
pCO atm.9.8 x 10-36.5 x 10-2 1.0 -1 1.0
hS 3.5 x 10 3.4 x 10-2 5.3 x 10 2.9 x 10-4
Approx. [w/o S] 6.2 x 10 3 9.6 x 10 2 5.3 x 10 5
-
The points B and C represent simultaneous equilibria between
the oxysulphide and two sulphides at 1500C. These univariant
points are only a function of temperature. The points E and
D represent the minimum sulphur contents or activities at which
oxysulphide and Ce2S3 can be formed, respectively, at pCO =
1 atm. Thus, carbon saturated hot metal cannot be desulphurized
by oxysulphide formation below hs~ 2.9 x 10 4 ([w/o S] ~ 5.3 x
10 5) at pCO = 1 atm. However, lower sulphur levels may be
attained by reducing the partial pressure of CO.
The conversion of CeO2-~Ce2O3-~Ce2O2S-~Ce2S3 is
illustrated in Figures 2a and 2b which show Ce2S3 and Ce2O2S
layers on a pellet of CeO2 (which first transformed to Ce2O3) ~-
on immersion in graphite saturated iron at ~1600C, initially
containing 0.10 w/o S, for 10 hours. The final sulphur content
was ~ 0.03 w/o S and the experiment was carried out under argon,
where pCOC~l atm.
The conversion of the oxide to oxysulphide and sulphide
13.
1077~8;~
is mass transfer controlled and, as in conventional external
desulphurization with CaC2, vigorous stirring will be required
for the simple addition process and circulation of hot metal
may be required in the 'active' lining process.
From Eigure 1 it is apparent that the external
desulphurization of graphite saturated iron is thermodynamic-
ally possible using RE oxides. For example the diagram
indicates that hot metal sulphur levels of ~0.5 ppm (point E)
can be achieved by cerium oxide addition even at pCO = 1 atm.
Desulphurization in this case will take place through the
transformation sequence CeO~Ce203-~Ce202S which required 2 moles
of CeO2 to remove 1 gm. atom of sulphur. The efficiency of
sulphur removal/lb. CeO2 added can, however, be greatly increased
by the formation of sulphides. 1 mole CeO2 is required per g.
atom of sulphur for CeS formation and 2/3 moles CeO2 for Ce2S3
formation. The theoretical CeO2 requirements for the removal
of 0.01 w/o S/THM for the various desulphurization products are
given below and expressed graphically in Figure 3.
. ~
PRODUCT lb CeO /0.01 w/o S.THM ft3CO/lb CeO2 ft3CO/0.01 w/o S.THM
? _ _ -
Ce2O2S 2.15 2.1 4.5
CeS 1.1 4.2 4.5
Ce3S4 0.8 4.2 3.4
Ce2S3 0.7 ~__ _ _ 3.0
The volume of carbon monoxide produced in ft CO/lb
CeO2 and ft3CO/0.01 w/o S.THM are also given in the above
table for each desulphurization product. For efficient
desulphurization the partial pressure of carbon monoxide should
be sufficiently low to avoid oxysulphide formation. For example,
1~ .
~07768Z
Figure 1 shows that oxysulphide will not form in a graphite
saturated melt until [w/o S] C 0.01 when pCO ~0.1 atm. It will
form however when [w/o S]~ 0.10 at pCO = 1 atm. Thus by
reducing the pCO in the desulphurization process to 0.1 atm.,
hot metal can be desulphurized to 0.01 w/o S with a CeO2
addition of 0.72 lb/0.01 w/o S removed for each ton hot metal.
The choice of the method of reducing the partial
pressure of carbon monoxide depends on economic and technical
considerations. However, in an injection process calculations
can be made for the volume of injection gas, say nitrogen,
required to produce a given pCO.
Thus: -
VN2 = Vc (l-pCO)/pCo
where
VCO is the scf of CO formed/lb CeO2 added
VN is the scf of N2 required/lb CeO2 added
and
pCO is the desired partial pressure of CO in atm.
The results of these calculations for Ce2S3 formation
are shown in Figure 4, which also shows the [w/o S] in equilibrium
with Ce2S3(s) as a function of pCO. From this figure it is
apparent that the volume of N2/lb CeO2 required to form Ce2S3
is excessive and if an injection process were used a balance
would have to be struck between sulphide and oxysulphide
formation. When, for example, hot metal is to desulphurize
from 0.05 to 0.01 w/o S at pCO = 0.2 atm., ~16 scf N2/lb CeO2
would be required for Ce2S3 formation and the sulphur content
would drop to 0.02 w/o. The remaining 0.01 w/o S would be
15.
1077~8~2
removed by oxysulphide formation. From Figure 3, lt can be
seen that _2 lbs of CeO2/THM would be required for Ce2S3
formation and 2 lbs for Ce2O2S formation giving a total require-
ment of 4 lbs CeO2/THM.
Calculations similar to the one above have been used
to construct Figure 5 where the CeO2 requirements in lbs/THM
are shown as a function of pCO.
When large volumes of nitrogen are used in an injection
process the heat carried away by the nitrogen, as sensible
heat, is not large but the increased losses by radiation may
be excessive. Injection rates with CaC2 for example are in
the order of 0.1 scf N2/lb CaC2.
Vacuum processing is an alternative method of reducing
the partial pressure of carbon monoxide. This is impractical
in hot metal external desulphurization but not in steelmaking
(see below).
Still another alternative approach to external
desulphurization using rare earth oxides is the use of active
linings which would involve the 'gunning' or flame-spraying of
HM transfer car linings with rare earth oxides. Here the
oxides would transform to oxysulphides during the transfer of
hot metal from the blast furnace to the steelmaking plant,
and the oxide would be regenerated by atmospheric oxidation
when the car was emptied. It is estimated that for a 200 ton
transfer car, conversion of a 2 mm layer (~ 0.080") of oxide
to oxysulphide would reduce the sulphur content of the hot
metal by ~ 0.02 w/o S. This process has the following advantages:
1) continuous regeneration of rare earth oxide by
atmospheric oxidation when the car is empty,
2) reaction times would be in the order of hours,
16.
-, ~ : :
- : :
1077682
31 the absence of a sulphur rich desulphurization
slag, and
4) the absence of suspended sulphides in the hot
metal.
The mechanical integrity and the life of an "active" lining
is, of course, critical and some pollution problems may be
associated with oxide regeneration by atmospheric oxidation.
With regard to steelmaking applications, vacuum
desulphurization could be carried out by an "active" lining in
the ASEA-SKF process and circulation vacuum degassing processes.
As an example of desulphurization of a stack gas,
assuming the following gas composition at 1000C.:
Component Vol.%
C2 16
CO 40
2 40
2 4
I~2S 0.3 3
(200 grains/100 ft .)
This equilibrium gas composition is represented by point A on
the diagram illustrated as Figure 1 where CO/CO2 = 2.5 and
H2/H2S = 133. This point lies within the Ce2O2S phase field
and at constant CO/CO2 desulphurization with Ce2O3 will take
place up to point B. At point B, H2/H2S ~104 and the concentra-
; tion of ~l2S is 0.004 vol.~ (~3 grains/100 ft3). Beyond this
point, desulphurization is not possible.
In the foregoing specification~ we have set outcertain preferred practices and embodiments of our invention,
however, it will be understood that this invention may be other- -~-
wise embodied within the scope of the following claims.
-
