Zinc Recovery by Chlorination Leach

1. Field of the Invention

The invention lies in the field of recovering metals from their ores and other metal containing material by first chlorinating the metals followed by final recovery of the metals from their chlorides.

2. Description of the Prior Art

Competition, the increasing necessity of using lower and lower grade ores, the requirement that metal recovery processes be essentially non-polluting, and other factors have created a demand for a reduction of the costs for recovering metals from their ores and other materials for the pollution-free processes. Conservation and re-use of reagents appears to be the most feasible area for reduction of costs.

U.S. Pat. No. 1,736,659, Mitchell, discloses a process for the recovery of metals from their sulfide ores in which the metals are first chlorinated and then selectively separated. In this process iron is separated from the desired zinc chloride by precipitation of the iron before the zinc recovery step. The disadvantage of this procedure is that substantial zinc losses will occur in the voluminous iron hydroxide precipitate. Further, Mitchell precipitates the zinc product from the original dissolution solution after various attempts at removing impurities from it. Such a system inevitably results in an impure zinc product. The present invention uses a zinc selective extractant by which the zinc is effectively cleanly separated from the dissolving solution without the necessity of prior precipitation of all other impurities. The present process thereby permits a higher recovery of higher purity zinc.

In the Mitchell process an electrolytic cell is used to generate from sodium chloride formed in the zinc precipitation step the base required to precipitate zinc. In the present process the electrolytic cell is used not only for this purpose but primarily to regenerate the sodium chloride stripping agent used to remove the zinc from the extractant so that additional reagent need not be added for stripping. Using sodium chloride regenerated in the process as the stripping agent and including the electrolytic cell in the stripping circuit to remove excess chlorine ions picked up by the stripping solution in stripping obviates the necessity for an additional stripping reagent and an external procedure for removing excess chlorine ions.

A process is disclosed for recovering zinc from materials in which it is contained in which zinc and other metals in the material are first chlorinated to form a leach solution of chlorides of the metals followed by recovery of lead chloride by crystallization, removal of trace metals such as copper, silver, arsenic, lead and bismuth, etc., by cementation, separation of the zinc chloride from the remaining ferrous chloride solution by tertiary amine or tributylphosphate extraction agents for the zinc chloride with the ferrous chloride being sent to an iron hydrolysis and chlorination step, the zinc chloride being stripped from the agent with an electrotyzed sodium chloride solution followed by the precipitation of zinc from the strip solution with sodium carbonate with the regeneration of sodium chloride which goes to a sodium chloride electrolytic cell for formation of chlorine and sodium hydroxide, the cell also regenerating the stripping agent by removing chlorine gas at the anode which is sent with the rest of the chlorine to the iron chlorination step, sodium hydroxide formed in the electrolysis step being converted to sodium carbonate which is sent to the zinc carbonate precipitation step. Part of the sodium hydroxide may be included in the stripping agent when the tertiary amine is used as an extractant to control the acidity of the amine extractant. Zinc is recovered from the precipitated zinc carbonate by calcination and the carbon dioxide formed is sent to the sodium hydroxide carbonation step. Chlorine from the electrolysis step is sent to the iron chlorination step in one modification of the invention while in an alternative modification in which chlorine gas is used for the chlorination step it is sent to the gas chlorination step.

An alternative procedure is the precipitation of zinc, as zinc hydroxide with sodium hydroxide from the cell thus eliminating the step of carbonating sodium hydroxide when the zinc is precipitated as the carbonate.

FIG. 1 is a flow diagram of the process of the invention,

FIG. 2 is a graph of a loading curve of zinc chloride on a tributylphosphate extractant, and

FIG. 3 is a graph of a stripping curve showing the effectiveness of stripping zinc chloride from a tributylphosphate extractant.

Reference will now be made to the flow diagram of FIG. 1 for a description of the process of the invention. The flow diagram and its description does not include the conventional equipment used in the various steps, such as, thickeners, filters, centrifuges, dechlorinaters, evaporaters, etc.

The feed can be any material which contains zinc, usually, a chlorinatable ore of zinc. The process can be used to recover zinc from scrap alloys of zinc. The invention is illustrated by its application to the recovery of zinc and other metals from a sulfide ore; however, it is by no means limited to this application as zinc can be recovered from various zinc containing starting materials. If the zinc is being recovered from an ore the latter will first be ground and concentrated. For this purpose of illustrating the operation of the invention as depicted in the flow diagram of FIG. 1 an example was selected in which the feed material was a concentrate of zinc ore which contained 24.4% zinc, 15.6% iron, 15.8% lead, 27.6% sulfur and 4.1 ounces of silver per ton, the example being described below.

One and two-stage leaches were performed by contacting varying amounts of the concentrate with 500 cc of ferric chloride lixiviant in conventional 1000 cc 3-neck flasks provided with paddle stirrers, reflexing condensers and heating mantles.

The lixiviant in all tests contained approximately 100 g/l ferric iron and 30 g/l ferrous iron. In the 1-stage leaches the amount of concentrate was varied so that 1, 50 and 200 per cent of stiochiometric ferric iron was available to react with the lead and zinc sulfides. It was found that ferrous chloride serves the function of holding the lead chloride in solution. The preferred concentration of ferrous chloride in the leach solution for holding all of the lead chloride in solution is in excess of about one molar. The leaches were performed at 100.degree.C for one half to four hours. The leaching resulted in conversion to the chlorides of 97.6% zinc, over 95% lead, 35-50% iron and 96% silver with the sulfur being converted to elemental sulfur which was removed in the tails.

An alternative procedure is the use of chlorine gas either alone or with some ferric chloride for the chlorination step. Following chlorination with the gas the remainder of the flowsheet is carried out. Chlorine gas from the anode is recycled to the gas chlorination step.

The next step in the flow diagram, after required filtration, is the recovery of lead chloride from the leach solution by crystallization. Lead chloride crystals were recovered from the 2-stage leach filtrate by cooling from the initial 80.degree.C to about 10.degree.C. The crystals contained 73.9% lead. Lead is recovered from the crystallized lead chloride by fused salt electrolysis to produce metallic lead and chlorine which latter is cycled to the iron chlorination step.

The filtrate from the lead chloride crystallization contained as impurities 1.09 g/l copper, 0.018 g/l antimony, 0.038 g/l arsenic and 1.52 g/l lead as well as the original silver content. These metal impurities were removed by conventional cementation procedures with metallic iron. The removal efficiency of the metal impurities by cementation was found to be 99.8% copper, 85% arsenic, 33% antimony and 9% lead. The treatment with metallic iron serves the additional purpose of reducing all of the iron in the solution to ferrous iron. Other metals than iron may be used for cementation of the trace metals and other means for removing them may be used. Other means for reducing the iron to the ferrous state or insuring that it is in the ferrous state and may be used. It is important that the solution which is contacted with the extraction agent later to remove zinc chloride be substantially free of ferric iron as the agents extract ferric iron which would contaminate the zinc product. One way to insure there is substantially no ferric iron in the metal depleted leach solution contacted by the zinc chloride selective extraction agent is to reduce the ferric iron with metallic iron or other reducing agent. However, the ferric chloride leach of the starting material can be readily conducted so that substantially no ferric iron exists in it.

The filtrate from the cementation step contained essentially ferrous chloride and zinc chloride with a minor amount of the metal impurities mentioned above. The next step is the separation of zinc chloride from the ferrous chloride with an extractant which is selective for the zinc chloride. Successful extractants for zinc chloride were found to be tributylphosphate and a tertiary amine (TriC.sub.8 -C.sub.1 amine) sold under the tradename "Agoden 364". Other tertiary amines can be used as extractants, such as, tri-laurylamine, tri-isooctylamine and tridecly amine. Other alkyl phosphates can be used, such as, the lower trialkylphosphates, including tripropylphosphate, dibutylphosphate and trioctylphosphate. By reference herein to zinc chloride in connection with loading and stripping from the agent is meant either molecular zinc chloride or an anionic zinc complex. The zinc complex as well as molecular zinc chloride can be stripped from the loaded extractant with water or sodium chloride solution. Conventional countercurrent extraction procedures were used.

Extraction isotherms were made for various extraction systems, FIG. 2 being a graph of an isotherm for two tributylphosphate systems and one Adogen 364 system. The use of an extractant containing 75% by volume of tributylphosphate to the organic solvent gives the best results. The organic solvent used was kerosene; however, other conventional organic solvents may be used as solvents for the extraction agents. A preferred range is from about 25 to 85 volume per cent of tributylphosphate to the organic. The isotherm shows that solvent loadings in excess of 30 l/g zinc were found to be possible and nearly complete extraction of the zinc can be achieved by using a number of extraction stages.

Zinc chloride was stripped from the loaded tributylphosphate and tertiary amine extractants with sodium chloride brine solution which is generated in the electrolytic cell as discussed hereinafter. Conventional stripping techniques were used. Other alkali metal chlorides can be used as stripping agents, such as, potassium chloride, by adjusting the system accordingly. FIG. 3 is a comparative stripping isotherm made from results obtained by stripping zinc from 75% tributylphosphate solvent with various concentrations of sodium chloride solution and water. As the isotherm shows, strip solutions containing 30 g/l of zinc were obtained using 2-3 molar sodium chloride solution for stripping.

The ferric iron that is entrained or otherwise extracted with the solvent will strip and contaminate the zinc solution. Accordingly, it is important to conduct the leaching so that substantially all of the iron is in the ferrous state or if this is not done to reduce as much iron as possible to the ferrous state and remove any remaining ferric iron as well as any bismuth, silver or cadmium before the loading step. The extractants do ont appreciably load ferrous iron. Any iron that is entrained in the stripping solution can be oxidized with chlorine and will precipitate with ferric hydroxide upon raising the pH to 3-4 with sodium carbonate. Only minute amounts of antimony, arsenic and lead will extract.

It was found that the teritary amines were effective as loading agents and can be stripped of zinc chloride with the sodium chloride strip solution coming from the electrolytic cell. When the amines are used as extractants the flow diagram is altered to use some of the sodium hydroxide from the cell to control the acidity of the amines for providing the best stripping efficiency.

As seen from the flow diagram, the ferrous chloride from which the zinc chloride was separated with the extraction agent is sent to iron hydrolysis and chlorination. By hydrolyzing part of the raffinate from the solvent extraction step exchange iron oxide is formed from the iron added for cementation and can be removed from this system. The remainder of the raffinate is chlorinated with chlorine from the electrolysis cell discussed below to ferric chloride which is returned to the leaching step. This permits use of the iron originally in the starting material as ferric chloride.

The solution from stripping contains essentially zinc chloride and sodium chloride. After further removal of iron by precipitation as ferrous hydroxide and cementing out trace impurities, such as, minute amounts of lead, cadmium etc. by zinc cementation with zinc dust the zinc chloride is sent to a zinc carbonate precipitation step where zinc is precipitated with sodium carbonate formed by carbonating sodium hydroxide produced in the electrolytic cell. After filtration zinc carbonate is calcined and otherwise treated to produce a high purity zinc product, some of which can be recycled to the zinc dust cementation step. As stated previously, an alternative procedure for the final recovery of zinc is to precipitate it as zinc hydroxide with sodium hydroxide from the cell and avoid the sodium hydroxide carbonation step.

The sodium chloride formed in the precipitation of zinc and that coming from the stripping step goes to the electrolytic cell where some of it is electrolyzed to produce chlorine at the anode and sodium hydroxide while some of it has its chlorine content reduced and then is recycled to the stripping step. The chlorine formed at the anode is sent to the iron chlorination step for chlorinating ferrous chloride to ferric chloride.

The cell used is commonly known as a chlorine-alkali cell or "Chloro-Alkali" cell and is of the type used for the commerical production of chlorine from sodium chloride. An anion ion exchange diaphragm which prevents the mixing of sodium chloride and sodium hydroxide in the cell is used. When sodium chloride is used to strip zinc chloride from the agents it was found that it picks up chlorine ion in increasing concentrations as it is recycled. Unless chlorine ion is continuously removed during recycling its concentration increases to the point were the solution does not effectively strip zinc chloride. Cycling the sodium chloride stripping solution through the cell where chlorine is continuously removed solves the problem.

Analyses showed that the process described is effective to recover 98% or more of the zinc contained in the starting materials as a high purity product.

The significant advantages of the invention as respects conservation of reagents are apparent from the above description and the accompanying flow diagram. First of all, all of the chlorine introduced into the system either by ferric chloride chlorination or gas chlorination is conserved and reused. None of it is eliminated from the system by the removal of any of the metal impurities. The chlorine in the lead chloride is recovered in the lead chlorine fused bath electrolysis and cycled to the iron chlorination step to regenerate ferric chloride. The chlorine in the metal chloride impurities is converted to ferrous chloride in the iron cementation step. The chlorine in the zinc chloride combines with sodium in the zinc carbonate precipitate to form sodium chloride which goes to electrolysis where the chlorine given off at the anode is used in the wet or dry chlorination of the metals in the starting material.

The ferrous chloride after the extraction step, containing most of the iron which was in the ore and the ferric chloride leaching agent, is sent to the iron chlorination and hydrolysis step for conversion to ferric chloride for re-use in the ferric chloride leach. The iron used for cementation is removed by hydrolysis. The result is that very little of the iron in the original ferric chloride leaching agent and the starting material is lost from the system.

Use of sodium chloride as a stripping agent is a distinctive advantage as the cell can be used to continuously reduce its chlorine content so that it does not become overloaded with chlorine in the stripping circuit. Production of sodium chloride in the zinc carbonate precipitation step provides a brine electrolyte for the electrolytic cell. Some of the zinc product can be reused in the zinc dust purification step and some of the carbon dioxide formed in the zinc calcination step can be used in the carbonation of sodium hydroxide to sodium carbonate.

The overall result of the process is that there is a maximum conservation and reuse of reagents wth very little addition of reagents required after startup.