HYDROMETALLURGICAL ZINC RECOVERY PROCESS AND APPARATUS

Abstract: There is provided a process for recovering a desired product from a residue and apparatus therefor. The process includes the step of recirculating either a residue formed in one stage of the process to an earlier stage of the process or the solid-waste residue to a later stage of the process. The invention uses these residues as a reagent in the step to which the residue is recirculated and thus reduces the amount of reagents which have to be added to the overall process thus reducing the cost of running the process. The earlier stage of the process is prior to a treatment step where acid is added to lower the pH of the waste being treated. The recirculation of the residue has the advantage for example of reducing the amount of acid required.

Description: HYDROMETALLURGICAL ZINC RECOVERY PROCESS AND APPARATUS The present invention relates to a process for recovering zinc or other desired products from waste and process residues, in particular, but not exclusively those blast furnace residues produced from zinc smelting and steel making (oxygen) operations. Waste and process blast furnace slags (residues) often contain substances which are valuable and/or difficult to dispose of, for example, containing hazardous metals and/or toxins. There are many known ways of removing such products from the solid residue, such as ion exchange, solvent extraction or precipitation processes. The TETRA Europe Ltd High Density Sludge (HDS) Direct Recycle and Thickened Recycle processes are particularly useful in the removal of such products by precipitation. Many types of residues have proved susceptible to treating in the previously known processes. For example see GB-A-2 128 597, GB-A-2 114 966, GB-A-2 084 555, GB-A-2 020 261, GB-A-2 010 235, GB-A-2 005 644 and GB-A-2 003 125. Certain types of residues have however proved to be unsuitable due to the characteristics of the residue, the expense of recovery, and/or the preparation of the residue required prior to treating by the previously known processes. Whilst recovery of metals from residues such as sludge has been successfully conducted, it has often proved uneconomic or otherwise impractical to recover products from many types of waste such as blast furnace slag or the slag from smelting operations which are characteristically vitreous in structure. Of course, the residue contains a large amount of various metals which are valuable and/or which can render the residue difficult to dispose of in an environmentally acceptable manner. Typically, such waste residue can contain up to 10% of zinc, but heretofore it has not proved practical to recover the zinc from the waste residue. However, the cost of recovery of the constituents often remains higher than the treatment and subsequent disposal of waste remains. Although many processes have been suggested for treatment of waste materials, in many cases it remains economically unviable to dispose of certain types of waste as outlined above . The present invention seeks to provide a method and apparatus to recover desirable constituents from a waste product . According to a first aspect of the present invention there is provided a process for recovering a desired product from a mixed-solid residue, comprising the steps of: a) treating the mixed-solid residue with an acid to make the desired product soluble in the leachate; b) leaching the desired product into the leachate from the mixed-solid residue; c) separating the desired product from the leachate by a precipitation process forming a precipitation residue and the desired product; d) recirculating at least part of the precipitation residue to step a) and/or adding at least some of the waste-solid residue directly to step c). There is thus provided a process for recovering a desired product from a residue, which includes the step of recirculating either a residue formed in one stage of the process to an earlier stage of the process or the solid waste residue to a later stage of the process. The invention uses these residues as a reagent in the step to which the residue is recirculated and thus reduces the amount of reagents which have to be added to the overall process thus reducing the cost of running the process. The earlier stage of the process is prior to a treatment step where acid is added to lower the pH of the waste being treated. The recirculation of the residue has the advantage for example of reducing the amount of acid required. Further, the recirculation step seems to assist the separation of desired products from the waste and generally improve the efficiency of the process, especially in the preferred and illustrated embodiments of the invention. According to a second aspect of the present invention there is provided a recovery apparatus for recovering a desired product from a mixed-solid residue, comprising: a) treatment means arranged to treat the mixed-solid residue with an acid; b) leaching means for leaching the desired product into the leachate from the mixed-solid residue treated in a) ; c) separating means for separating the desired product from the leachate arranged to form a precipitate residue and the desired product; d) recirculation means arranged to recirculate the precipitate residue to the treatment means and/or to recirculate at least some waste-solid residue directly to the separating means. The present invention thus provides a practical method of recovery from residues for many different metals and compounds and has particular utility when applied to zinc recovery. It is believed that the invention will make it viable to treat different types of waste. Normally, the residue is prepared to make it more susceptible to size reduction, to physical separation of desired products, and to solubilising of desired products in a leach solution. Advantageously, the preparation step comprises using the residue as a reagent in leachate neutralisation or pH adjustment and precipitation to selectively precipitate the desired products. Preferably, the refluxing of residue is continued back along the various stages until finally the residue is refluxed to the first step, e.g. to the size reduction plant. Over this multi-stage reflux recirculates the residue from the separation stages to the treatment stage. Normally, the prepared and size-reduced residue is treated with combinations of sulphuric, hydrochloric and nitric acids to make the desired products soluble in the leach solution. The residue may be entrained in water before passing through the acids leaching process; or residue may be reacted directly with the acids before passing through the water dilution process. Advantageously, the separating step c) comprises treating the leachate in a system selected from the group consisting of: a precipitation process; an ion exchange process; a solvent extraction process; and mixtures thereof. In a particularly preferred embodiment of the invention, the treatment step e) consists of treating the leachate in one or more high density sludge (HDS) processes operating in parallel and/or series. It is particularly advantageous to use residue in the HDS processes as the neutralising agent. The residue is separated from the HDS underflow sludge and refluxed to the prior neutralising stage and ultimately to the size reduction plant for final preparation before physical separation of desired products and leaching of desired products in the leaching solution. The act of reflux reduces the amount of neutralising agent required and the power required to size reduce the residue. Through destruction of the vitreous structure of the residue, the effectiveness of physical separation is also improved. The high calcium oxide content of the residue readily reacts with acid leach solution, and thus provides the additional benefits of acting as a neutralising agent. This leads to the breaking the internal structure of the vitreous residue which has the effect of reducing power consumption of the size reduction process and improve physical separation of desired products. The present invention will now be described with reference to the accompanying drawing in which: Figure 1 schematically illustrates a multi-stage recovery process according to a preferred embodiment of the invention. Figure 1 depicts a particularly preferred embodiment for recovering zinc and other materials from blast furnace slag. Waste residue is fed in to the process and in the illustrated embodiment the size of the residue particles are reduced in a size reduction step. The size reduction step reduces the typically 3mm uniformly-sized vitreous particles to about 50 - 200 micron particles size. Of course such a size reduction step may not be required in other embodiments depending on the initial size of the waste particles and the separation steps which are to be employed. The reduced-sized particles then undergo a physical separation step where products such as lead, gold and silver are recovered by density and/or magnetic means. Of course, the precise nature of this step will depend upon the constitution of the waste and this step may not be required in some cases. The waste then undergoes a series of leaching operation to leach desired products in to the leachate. water is used as the major constituent of the leachate. from the different separation operations. The leaching operation may be in batch or continuous stirred tank reactors, or by plug flow pipe reactor means. Unwanted solid materials are removed from the leachate by, for example, gravity means with residue dewatered to produce a cake. The leachate is then treated (neutralised) in a series of treatment stages to selectively precipitate desired products at each stage. The preferred illustrated embodiment shows a four treatment stage process, but the number and nature of the treatment stages will vary depending upon the constitution of the waste and the desirability/necessity of recovering the various constituents from the waste. In the illustrated embodiment each treatment stage is conducted at a different pH to form different precipitates containing the desired products at each stage. The first treatment stage uses calcium carbonate to adjust the pH of the leachate to about pH2 to precipitate calcium sulphate dihydrate (gypsum) . If alternatively a different compound or mixture of compounds was used instead of calcium carbonate then the precipitate may, of course, differ. The gypsum is recovered from the precipitate following dewatering and drying in a normal fashion. The second treatment stage uses lime to adjust the pH of the leachate to about pH6 to pH7 to precipitate principally ferric/ferrous hydroxide. If a different base was used instead of lime then an alternative iron salt may be formed. The iron product is recovered as a cake following dewatering of the sludge. In third treatment stage the pH of the leachate is further adjusted to about pH8 to pH9 with more lime to precipitate principally ferric hydroxide, aluminium hydroxide, copper hydroxide and arsenic. To aid arsenic recovery, ferric salts may be added to the leachate to co-precipitate iron with arsenic. If a different base was used instead of lime then an alternative precipitate may be formed. In the fourth treatment stage the pH of the leachate is further adjusted to about pHlO to pHll with more lime to precipitate principally zinc hydroxide. The zinc hydroxide may be recovered through dewatering of the sludge to produce a zinc hydroxide cake or alternatively the hydroxide can be combined with an acid to form a different salt such as zinc sulphate, zinc chloride or zinc bromide. It is preferred if in at least one of the leaching stages the initial waste residue (or size-reduced residue) is added (recirculated) to reduce the amount of reagent (base) which is required to precipitate the desired product in the particular separation stage. In the preferred, illustrated embodiment the at least some of the residue is added to the final (fourth) separation stage. From the fourth stage the residue is further refluxed upstream to the third separation stage and then refluxed to second separation stage and then on to the first separation stage. It is particularly preferred for the precipitate residue from the separating means, e.g. the underflow sludge of a HDS reactor, is recirculated back to the treatment stage. It is most preferred if the precipitate residue is recirculated and the waste-solid residue is added to the separating means as this maximises the befits of the present invention. The actual number of refluxes and the precise nature and direction of a reflux will depend on the particular recovery process being adopted. Generally however the reflux operation provides a way of significantly reducing the amount of reagent needed in some separation stages and therefore significantly improves the efficiency and cost- effectiveness of the overall process. It is preferred if at least one of the separation stageβ comprises a high density sludge (HDS) treatment and then the refluxed residue can be taken from the underflow sludge of the HDS process. Although the residue can be diluted with water prior to acid addition, the process is further advantageously enhanced by using the leaching technique of adding acid directly to the residue and subsequently diluting the product with water. This technique minimises the hydrolysis of silica in the residue and minimises the formation of aluminium silicate/ ' silicic acid compounds which may affect the subsequent leachate neutralisation and precipitation processes.

Claims: CLAIMS : 1. A process for recovering a desired product from a mixed-solid residue, comprising the steps of: a) treating the mixed-solid residue with an acid to make the desired product soluble in the leachate; b) leaching the desired product into the leachate from the mixed-solid residue; c) separating the desired product from the leachate by a precipitation process forming a precipitation residue and the desired product; d) recirculating at least part of the precipitation residue to step a) and/or adding at least some of the waste-solid residue directly to step c). 2. The process according to claim 1, further including a preliminary step of treating the mixed-solid residue to ensure the mixed-solid residue in step a) is smaller than a predetermined size. 3. The process according to claim 1 or claim 2, wherein more than one desired product is recovered from the process. 4. The process according to claim 1, claim 2 or claim 3, wherein the desired products are selected from the group consisting of lead, silver, gold, calcium, zinc, iron and arsenic. 5. The process according to any one of claims 1 to 4 , wherein zinc is recovered. 6. The process according to any one of the preceding claims, wherein the desired product, or desired products or some of the desired products are recovered in the form of salts. 7. The process according to any one of the preceding claims, wherein at least some of the recirculated precipitate residue is recirculated directly to the step a). 8. The process according to claim 2 or any claim dependent thereon, wherein at least some of the recirculated precipitate residue is recirculated to the preliminary step. 9. The process according to any one of claims 4 to 8, wherein the desired products are separated by treating the leachate in a process selected from the group consisting of: precipitation process; an ion exchange process; a solvent extraction process.; and mixtures thereof. 10. The process according to any one of the preceding claims, wherein the precipitation process, each precipitation process or some of the precipitation processes is/are a high density sludge process. 11. The process according to any one of the preceding claims, wherein the acid is added to the mixed-solid residue prior to dilution with water. 12. The process according to any one of the preceding claims, wherein the residue comprises a reagent to alter the pH in the precipitation process. 13. The process according to anyone of the preceding claims where there are several separation stages in step c) and the precipitate residue is refluxed to a prior separation stage of the process and this is repeated until final reflux to the waste-solid residue size reduction step or other first step. 14. The process according to claim 13, wherein the refluxed precipitate residue comprises the underflow sludge of a high density sludge reactor. 15. The process according to any one of the preceding claims wherein at least some of the waste solid residue is added directly to step c) and the precipitate residue is recirculated to step a) . 16. A recovery apparatus for recovering a desired product from a mixed-solid residue, comprising: a) treatment means arranged to treat the mixed-solid residue with an acid; b) leaching means for leaching the desired product into the leachate from the mixed-solid residue treated in a) ; c) separating means for separating the desired product from the leachate arranged to form a precipitate residue and the desired product; d) recirculation means arranged to recirculate the precipitate residue to the treatment means and/or to recirculate at least some waste-solid residue directly to the separating means. 17. The apparatus according to claim 16, further including size reduction means for reducing the size of the mixed- solid residue prior to treatment in the treatment means. 18. The apparatus according to claim 16 or claim 17, further including a plurality of leaching means. 19. The apparatus according to any one of claims 16 to 18, wherein the separating means includes a plurality of reactors arranged to separate one or more desired products from the leachate. 20. The apparatus according to claim 19, wherein the reactors are selected from the group consisting of: precipitation reactors; ion exchange reactors; solvent extraction reactors; and mixtures thereof. 21. The apparatus according to any one of claims 16 to 20, wherein the separating means comprises at least on high density sludge reactor. 22. The apparatus according to claim 21, wherein the precipitate residue is formed from the underflow sludge of the high density sludge reactor. 23. The apparatus according to any one of claims 16 to 22, wherein the recirculating means comprises means arranged to reflux precipitate residue from the reactor, each reactor or some of the reactors to an upstream reactor and finally to the treatment means . 24. The apparatus according to any one of claims 16 to 23 arranged to conduct the process of any one of claims 1 to 15 . 25. A process for recovering a desired product from a mixed-solid residue as hereinbefore described, or illustrated in the accompanying drawing. 26. A recovery apparatus as hereinbefore described or illustrated in the accompanying drawing.

Inventors: LLOYD ROBERT ALEXANDER, LLOYD, ROBERT, ALEXANDER
Applicants: LLOYD ROBERT ALEXANDER, TETRA EUROP LTD, LLOYD, ROBERT, ALEXANDER, TETRA EUROPE LIMITED
Application year: 1997
Main classification: C22B3
IPC codes: C22B3, C22B7, C22B19