Note: Descriptions are shown in the official language in which they were submitted.
10~31~1
This invention is concerned with a process for separat-
ing zinc and lead from ore concentrates in which these metals are
present in their sulfide form by using the sulfur atoms originally
combined with the æinc or lead.
The main object of this invention is to improve the ef-
ficiency of processes for recovering these metals from their con-
centrates so as to reduce the overall cost of producing such
metals and eliminate thermal and air pollution caused by prior
art processes.
The process whereby the above object is attained com-
prises smelting the ore under vacuum in the absence of carbon
dioxide and oxygen with a flux consisting of the tetrasulfides of
potassium or sodium, initially prepared by reacting a portion of
the ore with solid hydroxides of sodium and potassium, thereby
liberating said lead and zinc in the free state and forming the
pentasulfides of sodi~m or potassium, leaching the resulting slag
with water to dissolve said pentasulfides and allow;ng the leach
solution to contact air to form more hydroxide and lower sulfur
content polysulidQs for recycling.
A significant novel feature of the invention resides in
the preparation of the higher sulfides of sodium and potassium by
fusing zinc or lead sulfides with solid sodium or potassium hy-
droxides or polysulfides of sodium or potassium whereby use of
the sulfur atoms in the original ore is made to liberate more
metal.
Il 10~;931f~ 1
Because the literature in this field indicated that these
lead sulfides were not suluble in alkali hydroxides, the present ~ c'
;nvention is all the more unpredictable and unobvious, these
sulfides having been found soluble in fused potassium and sodium
hydroxides.
In the practice of the process of the invention, care
must be exercised to exclude 2~ C2 and water from the reaction
masses. Where water forms in any step of the process, it must be
continuously removed. The presence of water or of oxygen in the
system leads to the formation of oxides of zine and lead and oxy
compounds rather than of the free metals. Water also causes the
formation of hydrosulfides which decompose and of hydroxyl ions
causing some free zinc or lead to dissolve and form plumbates or
zincates. The presence of C2 is detrimental because it reduces
the period of time during which the solution of the ore with the
hydroxide flux remains liquid owing to the formation of carbonates
which increase the melting point of the melt. This in turn pre-
vents reactions from going to completeness.
The above outlined problems are avoided in accordance
with the invention by dehydrating the ore prior to mixing it with
the flux and by contacting the dehydrated ore with flux in the
absence of air and C02.
~0~931t~
The reactions are carried out in apparatus capable of
removing carbon d;oxide and oxygen from the reaction atmosphere.
One type of vessel used was a pure iron or nickel crucible with
screw-on air tight rings fastened to the outside upper surface of
the vessel with the crucible screwed into an inverted funnel-
shaped cover connected to a vacuum pump. Preferably, the pump
should have a rating of around 98%. The screw-on connection
operates by expansion of the rings when the bottom of the iron
crucible i5 heated thereby giving an air-tight seal. The melt
is stirred at 20-60 RPM by stirring means such as a stirrer.
After ceasing agitation, the temperature for the potas-
sium melt is increased to 500 C. and the sodium melt temperature
is increased to 550 C. The contents of the iron vessels contain-
ing the respective melts are transferred to a glass (transparent)
vessel under vacuum conditions. Upon cooling, the metals (zinc
or lead) are at the bottom (sp. gr of zinc is 7.1 while the zinc
oxide has a specific gravity of 5.6). The polysulfides ormed are
deposited în layers corresponding to ~heir specific gravity, the
higher sulfur content polysulfides are heavier and lie beneath
the lower sulfur content polysulfides. The lead is equivalent
in forming the separation layers but even more pronounced as both
the oxide and metal have higher specific gravity than the cor-
responding zinc and zinc oxide. Some of the ~inc will remain in
the iron melt vessel, adhering firmly to the sides. The lead
does not exhibit this property.
~o~
~n the initial step (using the hydroxides of potassium
and sodium) it is advisable to remove as great a quantity of the
lead or zinc oxide formed in the layer above the respective metals
as is possible.
This can be done by physical means, at least for the
eliminating
greater quantity ~-- the necessity of -3 micron filtration (hot)
at 500 C.), or by the alcohol or cold water washes, which dissol-~e
the lower sulfur content polysulfides of sodium or potassium. If
water is employed, in the potassium series (hydroxide or polysul-
fides), both the melt and the water must be cold.
The reactions on which the present process is based
can be summarized as follows:
1. 2ZnS + 2KOH 400C Zn + ZnO + K2S 2 + H2P
Atomic
Weight 194.76 112.22 6538 81.38 142.2 18
adjusted
to 130.48
Totals 306.9 306.9
2. 2PbS +2KOH 400C Pb + PbO ~-K2S2 + H2O
Atomic
Weight 478.4 112.22 207.2 223.2142.2 18
adj usted
to 130.48
Totals 590.62 590.62
3. 2ZnS +2NaOH 4 0C Zn ~ ZnO + Na2S2 + H2
Atomic
Weight 194.76 80 65.38 81.38 110 18
adj usted
to 105.2 `
Totals 274~76 274.76
,
~0 ~9 3 ~ ~
4. 2PbS ~2NaOH = Pb ~ PbO -' Na2S2 -~ H20
Atomic
Weight 478.4 80 207.2 223.2 110 18
adjusted
to 105.2
Totals558.2 558.2
The adjusted figures for the hydroxides represent a
correction to the absolute purity,
The potassium disulfide formed is soluble in absolute
ethyl alcohol. It was observed that some K2S3 and some K2S4
were formed. Some KOH remained and the over all empiric formula
corresponds to K2S2.
i The empirical formula Na2S2 represents a mix of NaOH,
Na2S2, Na2S3, Na2S4, and Na2S5. Some sulfur is also dissolved
in the NaOH. The existence of the disulfide and trisulfide of
sodium is problematical.
The sodium polysulfides are not as soluble as the potas-
alcohol in
sium polysulfides in/the di- and tri- stages and water is used to
dissolve the sodium polysulfides. Some sodium hydro-sulfide is
formed when the water is added. Both the di-hyrate and tri-
hydrate of sodium hydrosulfide are formed if the water is below
22C.
The potassium disulfide in alcohol solution is filtered
and evaporated to dryness. The sodium hydrosulfides are unstable
at low temperatures and decompose. Accordingly, when the next
step is performed the sodium hydrosulfide is not present.
10169318
The potassium disulfide is heated to above 470C. in
the iron vessel under the same reduced atmospheric pressure and
the melting point is reached when it is mixed with additional
zinc or lead sulfide.
5~ K2S2 + ZnS = Zn ~ K2S3
Atomic
Weight 142.33 97.38 65.38 174.40
Totals 239.7 239.7
6. K2S2 + 239.2 = Pb ~- K2S3
Atomic
Weight 142.33 239.2 207.2 174.40
Totals 381.5 381.6
The compound designated Na~S2 after being dried, is
heated to 550C. in an iron vessel under the 98% reduction of
atmospheric pressure, when the melting point is reached it is
mixed with additional zinc or lead sulfide.
7. Na2S2 ~ ZnS = Zn + Na2S3
Atomic !
Weight 144 97.38 65.38 176
Totals 241.38 241.38
8. Na2S2 ~ PbS = Pb + Na2S3
Atomic
Weight 144 239.2 207.2 176
Totals 393.2 383.2
After ceasing stirring of the melt, the temperature is
increased to above the melting point of zinc (419C.) or the
melting point of lead (325C.). Cooling allows the polysulfides
-7-
10~193~
of both sodium and potassium to separate above the lead or zinc
corresponding to the~r specific gravity. The lower sulfur content
polysulfides settle at the top, the higher sulfur content poly-
sulfides beneath them, the metals are on the bottom. The metals
can be decanted from the polysulfides, ,however, some zinc will
plate on iron vessels. No oxides are formed and the separation
after step 1 (the hydroxides) are simple and physical.
. The potassium trisulfide is soluble in absolute ethyl
alcohol. The tetrasulfide is much less soluble. Complete wash-
ing with alcohol (untiI no further solids remain after the evapor-
ation of the alcohol) permits a further water wash which dissolvec
- the sparingly alcohol soluble pentasulfide. Some pentasulfide is
formed apparently during the cooling of the melt.
The sodium trisulfide appears to contain a mixture of
disulfide and pentasulfide, some tetrasulfide, sodium hydroxide
and sulfur dissolved in the sodium hydroxide. The mixture is
water soluble, the water is filtered and evaporated.
These trisulfides are now used to treat additional zinc
and lead sulfides as shown:
9. K2S3 + ZnS = Zn + K2S4 at above 252C.
Atomic
Weight 174.4 97.38 65.38 206.4
Totals 271.7 271.7
10. K2S3 ~- PbS = Pb ~ K2S4
Atomic
Weight 174.4 239.2 207.2 206.4
Totals 413.6 413.6
106931b
11. Na2S3 + ZnS = Zn + Ns2S4 at 345C.
Atomic
Weight 142 97.38 65.38 174
Totals 239.38 239.38
12. Na2S3 ~ PbS = Pb ~ Na2S4
Atomic
Weight 142 381.2 2Q7.2 174
Totals 381.2 381.2
After ceasing agitation of the melt, the temperature is
increased to 325C. for the potassium, and the liquid polysulfides
can be poured off the zinc which is not melted at this temperature
! lead can similarly be treated at slightly less temperature (320C. .
It is advisable, after ceasîng the stirring to increase the tem-
perature briefly to reach the melting point of ~inc or lead in
order to have a button or one solid piece of metal which when
cooled to 325-320C. makes for an easy separation of the metals
from the polysulfides.
The potassium tetrasulfide is much less soluble in the
absolute alcohol than the potassium trisulf;de. The alcohol wash
can be used however.M~ny repeat washings are necessary to extract
all the alcohol soluble ingredients~ Some pentasulfide is formed
(the decomposition point of the pentasulfide is 300C. and this
extraction is carried out below this temperature). The pentasul-
fide is only sparingly ethyl alcohol soluble. ~ater can be used
as the solvent, however, the water should be cold, trisulfide de-
composes in hot water, and must be free of oxygen and carbon
lOf~1~3~
dioxide. The ~ater solution is filtered and evaporated to dry-
ness. The substances which remain are either hygroscopic
or deliquescent and heat must be used to attain dryness. The
heat used should be below 300C.
The sodium tetrasulfide formed is more uniform and
stable than the previous polysulfide forms. Again cold water is
used to dissolve the polysulfide and the solution is evaporated
to dryness.
The solution is evaporated to dryness to recover the
tetrasulfides of sodium or potassium while the zinc or lead is
recovered by filtration (prior to letting solution stand in air
for evaporation).
These tetrasulfides are used to treat further quantitie
of zinc and lead sulfides; the potassium at 210C; the sodium at
280C. as shown by the following equations:
13. K~S4 l ZnS = Zn + K2S5
Atomic
Weight 206.4 97.3865.38 238.4
Totals 303.8 303.8
14. K2S4 + PbS =Pb ~- K2S5
Atomic
Weight 206.4 239.2207.2 238,4
Totals 445.6 445.6
15. Na2S4 + ZnS = Zn ' Na2S5
Atomic
Weight 174 97.38 65.38 206
Totals 271.38271.38
~o~33~1
16. Na2S4 + PbS = Pb + Na2S5
Atomic
Weight 174 239.2 207.2 206
Totals 413.2 413.2
The pentasulfides formed in equations 15 and 16 are
leached with water. The aqueous leach solution is then left ex-
posed to air for eight hours to reconstitute the sodium or
potassium hydroxide~
It should be noted from the above that lead and zinc
are formed and recovered at each step by the process thereby im-
proving yields to around 99% basis starting sulfidic ore concen-
trate with the first two equat;ons yielding about 50% of the ore
as free metal
The potassium and sodium hydroxides used are reagent
grade.
The tetrasulfides of potassium are stable to 8Q0C. and
the sodium tetrasulfides are less stable but can be heated to
420C. The melting point of the tetrasulfide of potassium is
135C. and for sodium tetrasulfide the melting point is 275C.
Decomposition sets in just above this temperature, and must be
carefully reached in the case of sodium. With the potassium tetra
sulfide at low temperatures above 135C. the additional zinc or
lead sulfide is reduced to metals at these low temperatures with
the formation of the pentasulfide. The pentasulfide is liquid at
~o6 C. but decomposes at 300C. Thus to keep the melt liquid, a
temperature of over 206C. but below 300C. is necessary. After
the reduction of the zinc or lead to the metallic state, if the
:
10~i9~8
temperature is increased to the melting point of zinc (419C.)
the polysulfides break down to essentially tetrasulfide (stable
to 800C.) while the zinc or lead is collected at the bottom. In
the vacuum conditions of the mel~, the sulfur can not burn and
seems to both dissolve in the polysulfides and to come off as
molten sulfur, specific gravity 2.07 in the layer between the
cooled polysulfides and the metals. If the temperature is in-
creased to 445C. most of the sulfur is physically removable be-
tween the polysulfide layers and the metal layer at the bottom.
The recovered essentially tetrasulfide can be recycled with more
zinc or lead sulfide at this step. The sodium is not satisfactory
at this step, in regards to recycling or sulfur separation.
The invention is further illustrated by the following
specific examples, but it will be understood that the invention
is not limited thereto. The parts given are by weight unless
volumes of liquid are specified.
EXAMPLE I
194.76 parts of ZnS were preheated to substantially
dehydrate same. 130.48 parts of potassium hydroxide were heated
to the fus;ng temperature of around 400C. in an iron fusion pot
of the type above described under a vacuum of 98%. When the KOH
was completely melted, one third of the ZnS was added in less
than five minutes and the temperature was reduced to about 310C.
Another one-third of the ZnS was added and the temperatur~ was
reduced to 275C. and the remainder of the ZnS was added. The
melt turned yellow indicating the presence of X2S2. This com-
pound was recovered by repeatedly extracting the cooled melt with
-12-
10~1~31~
400 ml portions of absolute ethanol thereby giving 142.2 parts
of K2S2 Zn metal (65.38 parts) were separated physically and by
cold filtration. This amount of K2S2 was then heated to above
470C. in the same vessel under the same conditions as before.
When the K2S~ flux melted, 97.38 parts of ZnS were added and mixed
therewith. The melt turned brown yellow indicating the presence
of K2S3; 174.40 parts of K2S3 were collected by extracting with
500 ml portions of absolute ethanol and 65.38 of zinc metal were
collected by filtration. Next, 174.40 parts of K2S3 were heated
to above 252C. in the same apparatus under the same conditions
and ~7.3 parts of ZnS added thereto. The melt turned brown red
indicating the formation of K2S4. This material (206.4 parts)
was extracted with absolute ethanol 65.38 parts of Zn metal
were recovered, 206.4 parts of K2S4 were heated as before to
210C. and 97.38 parts of ZnS were mixed therewith to give a melt
which was orange indicating the presence of K2S5; 238.4 parts of
K2S5 were extracted with successive portions of 100 ml of water.
The resulting solution was lef~ in the open air for eight (8)
hours and 85% KOH were recovered by distilling the water; 65.38
parts of zinc metal were recovered by filtration.
EXAMPLE II
478.4 parts of PbS were preheated to substanially de-
hydrate same; 105.2 parts of NaOH were heated to the fusion
temperature of around 400C. in an iron fusion pot of the type
above described under a vacuum of 98~/o~ When the NaOH was com-
pletely melted, one third of the PbS was added in less than five
minutes and the temperature was reduced to about 310C. Another ;-
101j9~318
one-third of the PbS was added and the temperature was reduced to
275C. and the rema;nder of the PbS was added. Na2S2 was recover-
ed by repeatedly extracting the cooled melt with 400 ml. portior.s
of absolute ethanol thereby giving 1'0 parts of Na2S2.Iead metal
(207.2 parts) was separated by filtering and decanting. This
amount of Na2S2 was then heated to above 470C. in the same
vessel under the same conditions as before. When the Na2S2 flux
melted, 239.2 parts of PbS were added and mixed therewith; 176
parts of Na2S3 were collected by extracting with 500 ml. portions
of absolute ethanol and 207.2 parts lead metal were collected by
filtration. Next, 142 parts of Na2S3 were heated to above melt-
ing and 381.2 pakts of lead sulfide added to form Na2S4; 174
parts of Na2S4 were heated as before to 210C. and 23~.2 parts
of PbS were mixed therewith to give a melt containing Na2S5. 206
parts of Na2S5 were extracted with successive portions of 100 ml.
of water. The resulting solution was left in the open air for
eight hours and 85% NaOH was recovered distillîng the water;
207 parts of lead metal were recovered by filtration.
After the hydroxide treatment, above, whereby the melt
cools in layers corresponding to the respective specific gravitieC ,
and where the oxide of lead and zinc are formed, the further
separations of the polysulfides from the metals can be carried
out by simple decanting. When the final (pentasulfide) stage is
reached, use is made of the melting points and decomposition
points of the tetrasulfide and pentasulfide so as to reconstitute
essentially the tetrasulfide. (Th;s recycling of the material
is applicable to potassium.)
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