IE43820B1 - Extraction of zinc and lead from their sulfides - Google Patents

Description

This invention is concerned with a process for separating zinc or lead from ore concentrates in which these metals are present in their sulfide form by using the sulfur atoms originally combined with the zinc or lead.

Embodiments of this invention seek to improve the efficiency of processes for recovering these metals from their concentrates so as to reduce the overall cost of producing such metals and eliminate thermal and air pollution caused by prior art processes.

According to the invention there is provided a process for extracting zinc or lead from a sulfidic ore thereof, comprising smelting said ore under vacuum in the absence of water, C02 and oxygen with a flux consisting of fused solid sodium or potassium hydroxide to form lead or zinc and disulfides of sodium or potassium; separating the zinc or lead from the disulfides 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.

A significant feature of the processes of the preceding paragraph resides in the preparation of the higher sulfides of sodium and - 2 42820 potassium by fusing zinc or lead sulfides with solid sodium or potassium hydroxides or polysulfides of sodium or potassium whereby use of the sulfur atoms in the original ore is made to liberate more metal.

In the practice of processes embodying the invention, care must be exercised to exclude 02, C0z 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 zinc 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 CO2 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 increases the melting point of the melt. This in turn prevents reactions from going to completeness.

The above outlined problems are avoided in accordance with processes enbodying 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 COz. The flux is decarbonated prior to use, if necessary.

The reactions are carried out in apparatus capable of removing carbon dioxide and oxygen from the reaction atmosphere. One type of vessel air used was a pure iron or nickel crucible with screw-on/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 be able to evacuate to 98%. The screw-on connection operates by expansion of the rings when the bottom of the iron crucible is 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 in the case of the potassium melt is increased to 500°C and in the case of the sodium melt the temperature is increased to 550°C. The contents of the crucibles containing the potassium and sodium 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 formed are deposited in layers corresponding to their specific gravity, the higher sulfur content polysulfides are heavier - 3 9 and lie beneath the polysulfides which have a lower sulfur content. Some of the zinc will remain in the crucible, adhering firmly to the sides. The lead does not exhibit this property.

In 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. These oxides can be removed by filtering the reaction mass through - 3 micron perforated nickel or iron filters at around 500°C.

Alternatively, this can be done by alcohol or cold water washes, which dissolve the lower sulfur content polysulfides of sodium or potassium. If water is employed, in the potassium series (hydroxide or polysulfides), both the melt and the water must be cold.

The reactions on which processes embodying the invention are based can be summarized as follows:Atomic Weight 2ZnS + 2K0H 400°C Zn + ZnO + K2S2 + H20 194.76 112.22 65.38 81.38 142.2 18 adjusted to 130.48 Totals 306.9 306.9 Atomic Weight 2PbS + 2K0H 400°C Pb + PbO + K2S2 · 478.4 112.22 X 207.2 223. adjusted to 130.48 .2 142.2 Totals 590.62 590.62 Atomic Weight 2ZnS + 2NaOH 400°C Zn + ZnO + Na2S2+ H20 194.76 80 65.3881.38110 18 adjusted 105.2 to Totals 274.76 274.76 + 2NaOH —> 4. 2PbS Atomic Weight 478.4 Totals adjusted to 105.2 558.2 Pb + PbO + Na2S2+ H20 207.2 223.2.110 18 558.2 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 K2S3and some K2Si, were formed. Some KOH remained and the overall empiric formula corresponds to K2S2.

The empirical formula Na2S2 represents a mix of NaOH, Na2S2,Na2S3, Na2Si,, 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 potassium polysulfides in alcohol 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-hydrate and tri-hydrate of sodium hydrosulfide are formed if the water is below 22°C.

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 470°C. 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.

. K2S2 + ZnS — > Zn + K2S3 Atomic Weight 142.33 Totals 97.38 239.7 65.38 174.40 239.7 6. Atomic K2S2 + PbS — > Pb + K2S3 Weight 142.33 Totals 239.2 381.5 207.2 174.40 381.6 The compound designated NazS7after being dried, is heated to 550uC. 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. - 5 43 θ 7. Na2S2 + ZnS Zn + Na2S3 Atomic Weight 144 Totals 97.38 241.38 65.38 176 241.38 8. Na2S2 + PbS Pb + Na2S 3 Atomic Weight 144 Totals 239.2 393.2 207.2 176 383.2 After ceasing stirring of the melt, the temperature is increased to above the melting point of zinc (419°C) or the melting point of lead (325°C) Cooling allows the polysulfides of both sodium and potassium to separate above the lead or zinc corresponding to their specific gravity. The lower sulfur content polysulfides settle at the top, the higher sulfur content polysulfides beneath them, the metals are on the bottom. The metals can be decanted from the poTysulfides, however, some zinc will plate on iron vessels. No oxides are formed and the separation after step 1 (the hydroxides) is made simple and physical.

The potassium trisulfide is soluble in alsolute ethyl alcohol.

The tetrasulfide is much less soluble. Complete washing with alcohol (until no further solids remain after evaporation of the alcohol) permits a further water wash which dissolves 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: K2S3 + Zns —> Zn + K^at above 252°C.

Atomic Weight 174.4 Totals 97.38 65.38 206.4 271.7 271.7 - 6 4 3 8:-:0 . K2S, + PbS — >Pb KjSi, Atomic Weight 174.4 239.2 207.2 206.4 Totals 413.6 413.6 11. Na?S·, + ZnS—Zn + Na?S,,at 345°C Atomic Weight 142 Totals 97.38 239.38 65.38 174 239.38 12. Na2S3 + PbS — '> Pb + Na2Si, Atomic Weight 142 Totals 381.2 381.2 207.2 174 381.2 After ceasing agitation of the melt, the temperature is increased to 325°C. for the zinc, and the liquid polysulfides can be poured off the zinc which is not melted at this temperatures and which sinks. The lead can similarly be treated at a slightly lower temperature (320°C.). It is advisable, after ceasing the stirring to increase the temperature briefly to reach the melting point of zinc or lead in order to have a button or one solid piece of metal which when cooled to 325-320°C. 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 trisulfide. The alcohol wash can be used however, /-0 Many repeat washings are necessary to extract all the alcohol soluble ingredients. Some pentasulfide is formed (the decomposition point of the pentasulfide is 300°C. and this extraction is carried out below this temperature) The pentasulfide is only sparingly ethyl alcohol soluble. Water can be used as the solvent, however, the water should be cold, as the trisulfide decomposes in hot water. The water must be free of oxygen and carbon dioxide. The water solution is filtered and evaporated to dryness. The substances which remain are either hygroscopic or deliquescent and heat must be used to attain dryness. The heat used should be below 300°C.

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. - 7 43820 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 quantities of zinc and lead sulfides; the potassium at 210°C; the sodium at 280°C. as shown by the following equations: 13. KZS4 + ZnS -> Zn + K2S5 Atomic Weight 206.4 97.38 65.38 238.4 Totals 303.8 303.8 14. K2S4 + PbS —» Pb + K2S5 Atomic Weight 206.4 239.2 207.2 238.4 Totals 445.6 445.6 15. Na^S^ + ZnS —> Zn + Wa^Sg Atomic Weight 174 97.38 65.38 206 Totals 271.38 271.38 16. + 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 exposed 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 improving yields to around 99% basis starting sulfidic ore concentrate. Equations 1 and 2 yield about 50% of the ore as free metal.

The potassium and sodium hydroxides used are reagent grade.

The tetrasulfide of potassium is stable to 800°C. and the sodium tetrasulfide is less stable but can be heated to 420°C. The melting point of the tetrasulfide of potassium is 135°C. and for sodium tetrasulfide the melting point is 275°C. Decomposition sets in just above this temperature and must be carefully reached in the case of sodium. With the potassium - 8 4 3 S ·Ι Ο tetrasulfide at low temperatures above 135°C. 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 206°. but decomposes at 300°C. Thus to keep the melt liquid, a temperature of over 206°C. but below 300°C. is necessary. After the reduction of the zinc or lead to the metallic state, if the temperature is increased to the melting point of zinc (419°C.) the polysulfides break down to essentially tetrasulfide (stable to 800°C.) while the zinc or lead is collected at the bottom. Under the vacuum conditions of the melt, the sulfur cannot burn and seems to both dissolve in the polysulfides and to come off as molten sulfur; it has a specific gravity 2.07 in the layer between the cooled polysulfides and the metals. If the temperature is increased to 445°C. most of the sulfur is physically removable between the polysulfide layers and the metal layer at the bottom. The recovered potassium tetrasulfide can be recycled with more zinc or lead sulfide at this step. The recycling or sulfur separation of sodium tetrasulfide is not satisfactory at this step.

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 1 194.76 parts of ZnS were preheated to substantially dehydrate same. 130.48 parts of potassium hydroxide were heated to the fusing temperature of around 400°C. in an iron fusion pot of the type above described evacuated to 987,. 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 310°C. Another one-third of the ZnS was added and the temperature was reduced to 275°C. and the remainder of the ZnS was added. The melt turned yellow indicating the presence of K2S2. This compound was recovered by repeatedly extracting the cooled melt with 400ml portions of absolute ethanol thereby giving 142.2 parts of K2S2 and Zn metal (65.38 parts) which were separated physically and by cold nitration. This amount of K2S2 was then heated to above 470°C. in the same vessel under the same conditions as before. When the K2S2 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 - 9 43 8metal were collected by filtration. Next, 174.40 parts of K2S3 were heated to above 252°C. in the same apparatus under the same conditions and 97.3 parts of ZnS added thereto. The melt turned brown red indicating the formation of K2S^. This material (206.4 parts) was extracted with absolute ethanol; 65.38 parts of Zn metal were recovered, 206.4 parts of K2Si, were heated as before to 210°C. 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 K2S5were extracted with successive portions of 100 ml of water. The resulting solution was left 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 substantially dehydrate same; 105.2 parts of NaOH were heated to the fusion temperature of around 400°C. in an iron fusion pot of the type above described evacuated to 98%, When the NaOH was completely melted, one third of the PbS was added in less than five minutes and the temperature was reduced to about 310°C. Another one-third of the PbS was added and the temperature was reduced to 275°C. and the remainder of the PbS was added. Na2S2was recovered by repeatedly extracting the cooled melt with 400 ml. portions of absolute ethanol thereby giving 110 parts of Na2S2. Lead metal (207.2 parts) was separated by filtering and decanting. This amount of Na2S2 was then heated to above 470°C. 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 Ne.2S3 were heated to above melting and 239.2 parts of lead sulfide added to form NazSi(; 174 parts of Na2Si, were heated as before to 210°C. and 239.2 parts of PbS were mixed therewith to give a melt containing Na2Ss. 206 parts of Na^Sg 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 distilling the water; 207 parts of lead metal were recovered by filtration.

After the hydroxide treatment, above, as the melt cools in layers corresponding to the respective specific gravities, and where the oxide of lead and zinc are formed, the further separations of the polysulfides

Claims (13)

1. A process for extracting zinc or lead from a sulfidic ore thereof, comprising smelting said ore under vacuum in the absence of water, COg and oxygen with a flux consisting of fused solid sodium or potassium hydroxide to form lead or zinc and disulfides of sodium or potassium; separating the zinc or lead from the disulfides 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.

2. A process according to Claim 1, comprising smelting said ore under vacuum in the absence of water, COg 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 pentasulfides of said potassium or sodium.

3. A process according to Claim 1, comprising decarbonating a 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 lead or zinc and the disulfide of said potassium or sodium; separating said products; smelting said disulfide with additional sulfide ore under the conditions stated above to form more zinc or lead and the trisulfide of 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 tetrasulfide of sodium or 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.

4. A process according to any preceding Claim, 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 nickel or iron filters, at around 500°C.

5. A process according to any one of Claims 1 to 3, wherein any zinc or lead oxide formed by said smelting with said sodium or potassium hydroxide is removed by alcohol or water washes.

6. A process according to any preceding Claim, wherein said lead is separated from the polysulfides of sodium or potassium by heating the melt to about 320°C. - 12 43320 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. (This recycling of the material 5 is applicable to potassium). The process of the invention is particularly useful with galena and sphalerite ores. - 11 4 3 8.-:0

7. A process according to any preceding Claim, 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. 58. A process according to Claim 3, or any one of Claims 4 to 7 when appended to Claim 3, wherein said sodium or potassium pentasulfide is leached with water to form more hydroxide and lower sulfur content polysulfides; the resulting leach solution is exposed to air to reconstitute said sodium or potassium hydroxide and said hydroxide is mixed with additional ore thereby 10 making said process cyclic and continuous.

8. 9. A process according to Claim 2, or any one of Claims 4 to 7 when appended to Claim 2, further including the steps of leaching said sodium or potassium pentasulfide with water and exposing the leach solution to air to reconstitute said sodium or potassium hydroxide. 15

9. 10. A process according to Claim 9, wherein said leach solution is filtered to remove said lead or zinc metal prior to exposing said solution to air.

10. 11. A process as claimed in any preceding Claim, wherein said ores are galena and sphalerite. 20

11. 12. A process for extracting zinc or lead from a sulfidic ore substantially as herein described with reference to and as shown in the Examples.

12.

13. Lead or zinc when extracted from a sulfidic ore by a process according to any preceding Claim.