CA1069318A - Extraction of zinc and lead from their sulfides - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Sulfidic ores of lead or zinc such as galena or sphaler-ite are smelted under vacuum in the absence of CO2 and oxygen with a flux consisting of solid NaOH or KOH to free the lead or zinc as free metal and to form sodium or potassium disulfides which are used as flux in smelting additional sulfidic ores to liberate more metal and form still higher polysulfides of sodium or potassium also usable as flux up to the pentasulfide form. By the process of the invention, substantially complete use is made of the sulfur atoms initially combined with the lead or zinc to remove the sul-fur combined with said metals. Potassium hydroxide, its lower sul-fur content polysulfides can be reconstituted by exposing to air a leach solution of potassium pentasulfide.

Description

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.

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~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.

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 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 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.)

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for extracting zinc and lead from a sulfidic ore thereof, comprising smelting said ore under vacuum in the absence of water, CO2 and oxygen with a flux being at least one compound selected from the group consisting of hydroxides and polysulfides of sodium or potassium thereby liberating said zinc or lead in the free state and form a disulphide and/or the next higher polysulfide respectively of sodium or potassium, substantial-ly complete use of the sulfur atoms present in said sulfidic are thus being made to remove sulfur from said ore.

2. A process for extracting zinc and lead from a sulfidic ore thereof, comprising smelting said ore under vacuum in the absence of water, CO2 and oxygen with a flux consisting of fused solid sodium or potassium hydroxide to form lead and zinc and disulfides of sodium or potassium; separating said products and smelting said sodium or potassium disulfides under the above stated conditions with additional amounts of sulfidic ore to form further quantities of said metals and the next higher sulfide of said potassium or sodium and repeating said separation and smelting with each higher sulfide of potassium or sodium whereby substantially complete use of the sulfur atoms present in said sulfidic ore is made to remove sulfur from said ore.

3. A process for extracting zinc and lead from a sulfidic ore thereof, comprising smelting said ore under vacuum in the absence of water, CO2 and oxygen with a flux consisting of the tetrasulfide of sodium or potassium thereby liberating said zinc or lead in the free state and forming the penta-sulfides of said potassium or sodium whereby substantially complete use of the sulfur atoms present in said sulfidic ore is made to remove sulfur from said ore.

4. A process for recovering zinc or lead from sulfide ores thereof comprising: decarbonating potassium or sodium hydroxide flux; fusing said flux; adding thereto said ores and mixing said ores with said flux under vacuum in the absence of oxygen and of water thereby forming the disulfide of said potassium or sodium and said zinc or lead; separating said products;
smelting said disulfide with additional sulfide ore under the conditions stated above to form more zinc or lead and the trisulfide or sodium or potassium;
separating said products; smelting said trisulfide under the conditions above stated with additional sulfide ore to form more zinc or lead and the tetra-sulfide or sodium of potassium separating said products by extracting the reaction mass with absolute ethanol to obtain more zinc or lead and the tetrasulfide of potassium or sodium; smelting said tetrasulfide with additional lead or zinc sulfide to form zinc or lead and a pentasulfide of sodium or potassium substantially complete use of the sulfur atoms present in said sulfidic are thus being made to remove sulfur from said ore.

5. Process according to Claim 2, wherein any lead or zinc oxide formed by said smelting with sodium or potassium hydroxide is removed by filtering the reaction mass through -3 micron perforated nickle or iron filters, at around 500°C.

6. Process according to Claim 2, wherein any zinc or lead oxide formed by said smelting with said sodium or potassium hydroxide is removed by alcohol or water washes.

7. The process according to Claim 2, wherein said lead is separated from the polysulfides of sodium or potassium by heating the melt to about 320°C.

8. The process of Claim 2, wherein said zinc is separated from the polysulfides of sodium or potassium by heating the melt to around 325°C.

and pouring out the melted polysulfides leaving behind said zinc.

9. The process of Claim 4, wherein said potassium pentasulfide is leached with water; the resulting leach solution is exposed to air to reconstitute said potassium hydroxide and said potassium hydroxide is mixed with additional ore thereby making said process cyclic and continuous.

10. The process of Claim 3, further including the steps of leaching said potassium pentasulfide with water and exposing the leach solution to air to reconstitute said potassium hydroxide.

11. The process of Claim 10, wherein said leach solution is filtered to remove said lead or zinc metal prior to exposing said solution to air.

12. The process of Claim 2, wherein said ores are galena or sphalerite.