MXPA00003209A - Recovery of lead from flue dusts - Google Patents

Abstract

The present invention is concerned with a process for extracting and recovering lead or lead derivatives in high purity from various materials containing lead sulphate, and particularly copper smelter flue dusts. The present process also allows the substantially complete recovery or recycling of precious metals otherwise lost in flue dusts wastes.

Description

RECOVERY OF LEAD FROM CHIMNEY POWDERS FIELD OF THE INVENTION The present invention pertains to a process for extracting and recovering lead in byproducts of useful purity from foundry chimney powders. The present process also allows the substantially complete recovery or recycling of precious metals otherwise lost in chimney dust debris.

BACKGROUND OF THE INVENTION The copper pyrometallurgy can be summarized as a multiple-cap process where each stage is responsible for removing one or more impurities to finally obtain the pure copper metal. These impurities include, in addition to the non-metallic compounds, a variety of metals present in the feed material of the pyrometallurgical primary container of the melter. Some of these metals, such as silver and gold, are desirable because of their value and will be found in that medium in the copper produced from the smelting operation. Then these will be extracted during the refining of the later copper. Therefore, efforts are made to ensure that these precious metals are not lost during the pyrometallurgical process. During the smelting of copper concentrates and the conversion of matte, metals such as lead, mercury, arsenic, tin, cadmium, bismuth, silver, zinc, etc., are completely or partially volatilized, and therefore finished in the gases that come out of the oven. The gaseous current will also carry small particles sprayed from the slag and matte. Also, these particles contain precious metals such as silver and gold, as well as copper. In gas cleaning operations, most volatilized metals will be condensed and recovered as dust in electrostatic precipitators. The particle size of this fine powder is in the order of 2 microns or less, mainly containing sulfates, such as lead, copper and zinc sulfates, and oxides, such as oxides of tin and bismuth. Chlorides such as silver chloride may also be present at lower concentrations. The sprayed particles are on the other hand larger, typically from a few tens to a few hundred microns, and also recovered in the precipitators. The chimney powders therefore comprise a fine powder mixed with coarser particles. Although chimney dusts are an effective discharge to remove undesirable impurities from the copper mat, they nevertheless contain not insignificant amounts of precious metals, which means that their waste represents a significant economic loss, and can also create problems unacceptable environmental For these reasons, most industrial copper smelters recycle most of their chimney dust in the primary pyrro-metallurgical vessel, particularly that recovered from the precipitators. The non-recycled fraction of the chimney dust is usually sent directly to a lead smelter, and can sometimes be treated before being sent there. This operation constitutes important costs for copper smelters. The treatment of lead sulphate is actually expensive because heavy fines are imposed by lead refiners if impurities such as arsenic, bismuth, cadmium, copper, tellurium, chlorine, etc., are present in the powders used as materials of feed for lead smelters.

Chimney powders can be treated in a pyrometallurgical manner, such as for example in U.S. Patent No. 4,013,456 or U.S. Patent No. 5,256,186, but hydrometallurgical methods are much more preferred. It is well known that metals such as copper, zinc, cadmium and arsenic are already soluble in water in different degrees. The United States patent No. 4,891, 067 proposes the selective solubilization of lead, arsenic, molybdenum and antimony in a caustic medium. The use of sulfuric acid to increase the dissolution of copper and other impurities is conventionally done by many smelters around the world, and several publications acknowledge this fact. See, for example: Piret in the 3rd International Symposium on Recycling of Metals and Engineered Materials, 1995, 189-214; Minoura et al. In Metallurgical Review of the MMIJ, 1984, 1 (2), 138-156; Shibasaki et al. In Hydrometallurgy, 1992, 30, 45-57; and Ino et al. in Proceedings of Copper 95 International Conference, 1995, 3, 617-627. Under certain higher sulfuric acid concentrations, a significant amount of bismuth is solubilized, which can be recovered by dilution, carburization or neutralization, as illustrated in Hanks et al. In Transactions of IMM, Section C, 1979, 99-106. The addition of chloride ions, either through sodium chloride or hydrochloric acid, solubilizes the bismuth as well as a significant amount of the silver present in the chimney dust. See Asahina et al. In Proceedings of the joint Meeting MMIJ-AIME, 1977. 2, 856-874; and Hydrometallurgy, 1985. 14, 93-103. U.S. Patent No. 5,443,622 is interested in a process that uses a liquor containing chlorides and fluorides to solubilize bismuth and recovery of cadmium, arsenic and bismuth. In all of the above methods and processes, the lead present remains in the form of impure lead sulfate which can be either recycled to the primary pyrometallurgical vessel of the copper melter or sent to a lead smelter with all of the economic disadvantages described above. In the treatment of lead-containing sulfur ores, as for example in U.S. Patent No. 4,266,972 and in U.S. Patent No. 4,410,496, lead and most other metals in the ore are leached under over oxygen pressure in an acid medium containing chloride ions. The subsequent recovery of the metals in the stock solution is carried out by several known techniques. U.S. Patent No. 4,372,782 discloses a method for recovering lead and silver from minerals and side products containing lead sulfate. In this method, the lead is solubilized in an acid solution having a pH of about 1.5 and highly concentrated in chloride ions, for example, 269 g / L NaCl. Calcium chloride is used to remove excess sulfate and increase the solubility of lead to approximately 18 g / L. Following a liquid / solid separation, lead chloride oxide is precipitated by adding lime. Subsequently, the oxychloride is calcined at 400 ° C with lime to produce the calcium plumbate which will be sent to a lead smelter. This process does not allow the elimination of bismuth from lead, and because of the low solubility of lead sulfate in the medium, that is, 18g / L, a significant capital investment is required to be able to treat tens of tons of powders. chimneys rich in lead that are produced daily in conventional copper smelters. Therefore, it would be highly desirable to develop an efficient process for the hydrometallurgical treatment of the chimney powders produced from the smelters. Such a process should allow a substantially complete extraction and recovery of lead and precious metals economically. In addition, the lead and lead derivatives obtained must be of good purity to remove the fines charged by the lead smelters to refine this product.

BRIEF DESCRIPTION OF THE INVENTION According to the present invention, now there is provided a process for extracting and recovering substantially all of the lead from solid materials containing lead in the form of lead sulfate, and more particularly the chimney powders generated from the smelters of copper. More specifically, the process comprises the steps of: a) leaching the material containing lead sulfate with an acid solution containing a sufficient concentration of chloride ions to solubilize the metals in the solution and convert the solid lead sulfate to lead chloride solid, with the proviso that the concentration of the chloride ions is not sufficient to solubilize substantial quantities of lead, and the pH of the acid solution does not exceed 0.5; b) Perform a solid / liquid separation to separate a solid fraction and a liquid fraction of acid; c) Leaching the solid fraction obtained in step b) with water to selectively dissolve the lead chloride; d) Recover a lead chloride solution from step c) after solid / liquid separation if necessary, and e) Optionally convert lead chloride to lead metal or other lead derivatives, so substantially all The lead present in the material containing lead sulfate has been extracted from it. In a preferred embodiment, the acid solution is a solution of sulfuric acid containing sodium chloride as the source of the chloride.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a schematic flow sheet of the present process; Figure 2 illustrates the modification to the flow sheet of Figure 1 when calcium salts are used in the neutralization step with a leaching solution of HC1; and Figure 3 illustrates the modification to the flow sheet of Figure 1 which allows the production of pure lead chloride.

DETAILED DESCRIPTION OF THE INVENTION In the process of the present invention, the chimney powders obtained from the copper smelting operations are hydrometallurgically treated to recover the lead in a commercially advantageous manner. The process also allows the substantially complete recovery of all precious metals present in the flue dust. Although the present process is particularly advantageous for chimney dusts produced from copper smelters, it should be noted that this can be applied to any material that contains lead in the form of lead sulfate or converted to it before the process is carried out. Because of the presence of particles sprayed with slag and matte in the chimney powders recovered from the electrostatic precipitators, the next optional preliminary treatment in the chimney powders can be carried out before proceeding with the present process. This preliminary stage is justified by the fact that the particular sprinkles of slag and matte are thicker than the particles of regular flue dust. Typically, the preliminary process comprises mixing the powder in water to form a suspension which is subsequently subjected to gravimetric separation to produce a concentrate of slag and slag particles, and a residue of smaller particles. This concentrate is recycled to the primary pyrometallurgical vessel. The residue is optionally acidified, preferably with sulfuric acid, at a pH of between about 0.7 to 2.5, to further dissolve the copper, zinc, cadmium and arsenic. The suspension is then filtered, and the filtrate, after recovering the copper therefrom in a conventional manner, such as iron cementation, is transferred to the effluent treatment plant. The filter cake contains substantially all of the lead in the form of lead sulfate, as well as substantially all of the bismuth, tin and a larger fraction of silver originally present in the flue dust. It can also contain significant amounts of gold and copper, as well as low concentrations of arsenic, zinc, cadmium and iron. This is the filtration paste that is treated according to the present process. It should be noted that the waste can also be treated according to the present invention just after the gravimetric separation. Also, chimney powders can be treated directly according to the present process without going through the preliminary steps described above. In the present process, the suspension of lead sulfate is leached with an acid solution containing a sufficient concentration of chloride ions to solubilize the bismuth and convert the lead sulfate to lead chloride. The pH of this solution is preferably between 0 to 0.5 to ensure efficient conversion of lead sulfate to lead chloride. Both of the concentrations of the chloride and acid ions will vary depending on the percentage of solid particles of the flue dust in the leachate rector and the concentration of lead sulfate in the flue dust. However, and this is critical to the process, the concentration of chloride ions should be sufficient to allow the conversion of lead sulphate to lead chloride, but low enough to avoid solubilization of lead in the form of a soluble PbC13 complex. . According to the present process, the suspension contains, after the acid leachate, a solid fraction of lead chloride, gold as well as other insoluble metals. Following the liquid / solid separation, the metals dissolved in the liquid phase are recovered in a conventional manner, such as by hydrolysis, cementation, neutralization or ion exchange. With respect to the solid fraction, it is leached with hot water to selectively solubilize the lead chloride, and the rest of the solid fraction is then returned to the primary pyrometallurgical vessel. The leaching solution substantially contains only lead chloride, which can be recovered as metallic lead by iron cementing or by electrolytic recovery, or as other lead salts of useful purity after conventional conversion treatment. If the chimney powders contain silver in the form of silver chloride, the conditions according to the present process to convert the lead sulphate of lead chloride will allow this silver salt to solubilize. The silver can subsequently be recovered with the other metals such as bismuth, copper and tin in a conventional manner. Alternatively, if one prefers to keep the silver in the solid phase, cementing, burr or copper scrap can be done simultaneously with the leaching stage. The cemented silver will therefore end up in the solid phase with the metallic copper and the unreacted lead chloride, while the solubilized copper free from the cementing reaction if it is recovered from the solution in a conventional manner. The additional features, objects and advantages will become apparent from the following detailed description of the preferred embodiments of the present invention taken in combination with the accompanying drawings. This description should be interpreted as illustrative of the invention instead of limiting its scope.

Referring to Figure 1, the chimney dusts 1, whether or not treated in accordance with the preliminary treatment described above, or in fact any material containing lead sulfate, are leached in two with a solution containing hydrochloric acid a a temperature of 20 to 90 ° C, and preferably between 30 and 60 ° C under atmospheric pressure. The chloride source can be any chloride salt soluble in acid, as long as it does not interfere with the reaction that is caused by the cation. Sodium chloride is very preferred because of its availability and low cost. As stated above, the concentration of the acid and the chloride ions therein must be such that the bismuth as well as other elements such as copper, zinc, cadmium, arsenic, etc., in the chimney powders are solubilized while substantially all Lead sulfate has been converted to lead chloride without the solubilization of lead in the form of PhC13. This solid to solid conversion can be explained as follows. Lead sulfate is only soluble very slightly in acid solutions. Because the acid solution contains an excess of chloride ions, as soon as it dissolves a molecule of lead sulfate, the free lead ion reacts with the chloride ion in solution to form the lead chloride which, because of the experimental conditions provided, precipitates instantly. Therefore, although it can be observed that nothing happens in the reaction medium since both of the lead sulfate and the lead chloride are white powders, that is where the reaction is in fact taking place. After solid / liquid separation 3, a solution 11 containing various metals of interest, such as bismuth, copper, tin and optionally silver is obtained, as well as a solid containing mainly lead chloride and other elements such as gold, and optionally silver. The solid is leached with hot water at 4 to selectively solubilize substantially all of the lead chloride. Hot water is preferred over cold water for obvious reasons of solubility, the solubility of the lead chloride being higher in the former, ie 26 g / L at 80 ° C instead of 12 g / L at 30 ° C. Since substantially all of the original compounds and water-soluble elements have already been removed, the lead chloride is therefore selectively solubilized in hot water. Accordingly, lead dissolves in the free Pb + state, rather than the complex form of PbC13. "This is a significant advantage over known processes that use direct solubilization of lead sulfate in highly concentrated chloride medium. , because the undesirable precipitation of the lead oxychlorides is avoided The leaching step 4 is followed by an additional liquid / solid separation 5 wherein the solid 12 containing the precious metals is recovered and recycled to the primary pyrometallurgical vessel. Liquid almost exclusively contains lead chloride dissolved in water, and lead can therefore be recovered in that form, or converted to elemental lead, lead oxide, lead hydroxide, lead carbonate, lead chloride or any other form commercial in any conventional manner, for example in 6, a simple neutralization with lime, magnesium hydroxide, sodium hydroxide or any other basic material, below the pH of 9, leads to the precipitation of lead hydroxide, which can be recovered by a liquid / solid separation. 7. Sodium carbonate can also be used, and causes lead to precipitate in the form of basic lead carbonate. Lead can also be recovered by other conventional methods such as cementation. A final solid / liquid separation 7 gives the desired lead compound, and the chloride rich solution can optionally be subjected to a concentration or evaporation step 10 and recycled to stage 2 of the process through a mixing tank 9 which also receives the recycled fraction of the leaching solution. As illustrated in Figure 3, the cooling of the lead chloride solution in 15 before neutralization 6 will substantially crystallize the pure lead chloride 17 which can be recovered by a simple solid / liquid separation step 16. This crystalline salt of lead chloride can be to dissolve in hot water and used as a starting material to produce the lead metal with high purity or lead products through stages 6 to 8 of Figure 1. The use of lime to neutralize the chloride solution of Lead is economically advantageous, but the presence of calcium ions in the mixing tank 9 will result in the precipitation of calcium sulfate. If the calcium sulphate is not removed from the solution, it will be partially leached by the hot water during stage 4 with the lead chloride, and will cause the precipitation of the lead sulfate, which can be recycled to the primary pyrometallurgical vessel with the precious metals. To minimize these lead losses, as illustrated in Figure 2, the formation of calcium sulfate should be maximized in the mixing tank 9, for example by adding sulfuric acid, and filtering the newly formed calcium sulfate at 13 and optionally at 14, and transfer it to an effluent treatment plant. Typically in a batch process, a 200 g / L sulfuric acid solution containing 200 g / L of NaCl is good. In a continuous process, the concentration of sulfuric acid remains unchanged but the NaCl concentration can be reduced to 150 g / L. Preferably, a mass ratio (mass of H2SO4 / mass of PbSO4) and (mass of NaC1 / mass of PbSO) of about 1.6 is maintained for a particle density of chimney powders in the suspension of about 12%. If the density of the particles in the suspension is higher, ie 17%, a mass ratio of approximately 1.2 is maintained, and if the density is lower, ie 9%, a mass ratio of 1.8 is maintained. In general, a mass ratio between 1.0 and 2.0 is highly preferred, but may be higher or lower depending on the density of the particles of the flue dust in the acid solution. Further preferred experimental conditions include a temperature of the acid leach solution of about 50 ° C and a residence time of the particles of the flue dust in the leaching solution of about 30 minutes to about 2 hours. Under these conditions, substantially all of the lead sulfate is converted into lead chloride, and approximately 95% of the bismuth and 95% of the silver dissolve. Similar results have also been obtained at various densities by replacing sulfuric acid and NaC1 in step 2 with a hydrochloric acid solution 80 g / L. In this case, the hydrochloric acid solution therefore acts as both of the chloride ion supplier as the acid solvent. Therefore there are much lower sulfate ions in the solution, which results in a shorter reaction time because the effect of inhibition of the sulfate ions in the solution is reduced. As a result, the reaction can be carried out at room temperature, generally within one hour. Mixtures of hydrochloric acid and sodium chloride can also be used. For example, a leaching solution containing 55 g / L hydrochloric acid with 90 g / L NaCl works well in the batch process for a suspension containing approximately 12% chimney dust particles. With the present process, the lead is recovered from the chimney dust in a yield exceeding 90%, and the products obtained after the neutralization stage do not contain dangerous impurities and are fully compatible with lead pyrometallurgy, this way minimizing later foundry costs and representing a general economical advantageous operation because: - the lead being of high degree, this can be sold directly to buyers after conversion to the required product (lead, lead oxide, lead hydroxide, etc.), or sent to a lead smelting operation without the need to pay rigorous fines; substantially all the precious metals present in the chimney powders are either recovered or recycled to the primary pyrometallurgical vessel of the smelter. Therefore, nothing is lost; The process requires little investment and the reagents used cheaply. For example, in a preferred embodiment, the leaching solution comprises sodium chloride dissolved in sulfuric acid. Because most smelters have sulfuric acid plants, they can pull any amount of acid required to carry out the process, and sodium chloride is readily available and cheap. The discharge fraction 11 of the leaching solution that has been enriched with the recycle can be treated by carburizing, hydrolysis, neutralization or any other conventional technique for recovering metals having a commercial value, such as silver, bismuth, tin, indium. , zinc and copper present there. +++ Table I presents the results of the lead analysis for dried lead products (at 105 ° C) obtained according to the process of the present invention. As can be seen, high grade quality products have been obtained.

TABLE I Lead Content for Lead products PbO products «> Pb (OH) 2 2PbC03 »Pb (OH) 2 PbCI2% lead (measured) 88.8 81.9 74.8% lead (theoretical) 89.2 80.1 74.7 While the invention has been described in relation to the specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to protect any of the variations, uses and adaptations of the following invention, in general, the principles of the invention and including said deviations from the present disclosure as they occur within known and customary practice within the art to which the invention pertains, and as it may be applied to the essential features set forth hereinbefore, and as follows within the scope of the appended claims.

Claims (15)

1. A process for extracting and recovering substantially all of the lead from the lead sulfate containing material, the process comprising the steps of: a) leaching the lead sulfate containing material with an acid solution containing a concentration of chloride ions sufficient to solubilize metals in the solution and convert the solid lead sulfate to solid lead chloride, provided that the concentration of chloride ions is not sufficient to solubilize substantial amounts of lead, and the pH of the acid solution is not higher than 0.5; b) conducting a solid / liquid separation to separate a solid fraction and an acid liquid fraction; c) leaching the solid fraction obtained in step b) with the water to selectively dissolve the lead chloride; d) recover the lead chloride solution from step c) after solid / liquid separation if necessary; and e) optionally converting the lead chloride to lead metal or other lead derivatives; whereby substantially all the lead present in the material containing lead sulfate has been extracted therefrom.

2. A process according to claim 1, wherein the acid solution is a solution of sulfuric acid or hydrochloric acid.

3. A process according to claim 2, wherein the solution of sulfuric acid or hydrochloric acid contains sodium chloride.

4. A process according to claim 3, wherein the mass ratio of (mass of sulfuric acid / mass of lead sulphate) and of (mass of sodium chloride / mass of lead sulfate) varies from 1.0 to 2.0. .

5. A process according to claim 1, wherein the lead sulfate material is chimney dust obtained from foundry operations.

6. A process according to claim 5, wherein the casting operation is a copper smelting operation.

7. A process according to claim 1, wherein the acid solution obtained after the solid / liquid separation in step b) is treated to recover the metals present there.

8. A process according to claim 1, wherein the water temperature is at least 20 ° C.

9. A process for extracting and recovering substantially pure lead or lead derivatives from metallurgical chimney powders containing lead sulfate, the process comprising the steps of: a) leaching chimney dust with a sulfuric acid solution containing a concentration of chloride ions sufficient to solubilize metals in the solution and convert the solid lead sulfate to solid lead chloride, provided that the concentration of chloride ions is not sufficient to solubilize substantial amounts of lead, and the pH of the sulfuric acid is not greater than 0.5; b) conducting a solid / liquid separation to separate a solid fraction and a sulfuric acid fraction; c) leaching the solid fraction obtained in step b) with water to selectively dissolve the lead chloride therefrom; d) recovering the lead chloride solution from step c) after solid / liquid separation; and e) optionally converting the lead chloride to lead metal or other lead derivatives; whereby substantially all the lead present in the flue powders has been removed therefrom.

10. A process according to claim 9, wherein the chimney powders are obtained from copper smelting operations.

11. A process according to claim 9, wherein before carrying out step a), the chimney dust is subjected to the following preliminary treatment: i) the chimney powders are mixed in water to form a suspension which is subjected to gravimetric separation to produce a concentrate of particles sprayed with slag and matte, and a residue of smaller particles; ii) the sprayed particle concentrate is returned to the primary pyrometallurgical vessel, and the smaller particles are treated more according to the process of claim 9.

12. A process according to claim 9, wherein the acid solution obtained after the solid / liquid separation of step b) is treated. to recover the metals present there.

13. A process according to claim 8, wherein the water temperature is at least 20 ° C.

14. A process according to claim 9, wherein the sulfuric acid solution contains sodium chloride.

15. A process according to claim 14, wherein the mass ratio of (mass of sulfuric acid / mass of lead sulfate) and of (mass of sodium chloride / mass of lead sulfate) varies from 1.0 to 2.0. .