TREATMENT OF ZINC LEACH RESIDUE

Abstract

According to the present invention there is provided a method for treating a zinc leach residue comprising the steps of: adding the zinc leach residue and a sulfide material comprising copper and flux to a furnace having a molten bath therein; operating the furnace to produce a matte comprising copper and a slag comprising zinc; separating the matte from the slag; and recovering zinc from the slag. The method preferably comprises the additional step of recovering the copper and/or other precious metals such as silver and gold, from the matte.

Claims

1. A method for treating a zinc leach residue comprising the steps of: adding the zinc leach residue and a sulfide material comprising copper and flux to a furnace having a molten bath therein; operating the furnace to produce a matte comprising copper and a slag comprising zinc; separating the matte from the slag; and recovering zinc from the slag, wherein the matte formed in the furnace comprises between about 40 to 75% copper.

2. A method according to claim 1, further comprising the step of recovering the copper from the matte.

3. A method according to claim 1, further comprising the step of treating the slag in a slag fumer or a slag fuming furnace to recover the zinc therefrom.

4. A method according to claim 1, further comprising the step of treating the matte in a copper smelter to recover copper therefrom.

5. A method according to claim 4, further comprising the step of recovering one or more precious metals from the final product of the smelter.

6. A method according to claim 5, wherein the one or more precious metals comprise silver and gold.

7. A method according to claim 1, wherein the furnace comprises a top-blown submerged-combustion lance (TSL) furnace.

8. A method according to claim 1, wherein the furnace is operated at an oxygen partial pressure between about 10.sup.9.5 and 10.sup.7.5 atm.

9. A method according to claim 1, wherein the furnace is operated at an oxygen partial pressure of about 10.sup.8.5 atm.

10. A method according to claim 1, further comprising the step of adding air, oxygen, and/or oxygen-enriched air to the furnace.

11. A method according to claim 1, wherein off gases from the furnace contain sulfur dioxide and are sent to an acid plant to produce sulfuric acid therefrom.

12. A method according to claim 1, further comprising the step of adding one or more fluxes to the furnace.

13. A method according to claim 12, wherein the one or more fluxes comprise silica, limestone, or another source of CaO.

14. A method according to claim 12, wherein the fluxes and feed added to the furnace are selected or controlled to produce a slag with a zinc content in the range of about 10-20 wt. %.

15. A method according to claim 1, wherein the slag has a CaO/SiO.sub.2 ratio of between about 0.1 to 0.3.

16. A method according to claim 1, wherein the slag has a ratio of SiO.sub.2/(Fe+Zn) of between about 0.6 to 0.8.

17. A method according to claim 1, wherein the furnace is operated at a temperature of about 1100 C. to 1250 C.

18. A method according to claim 1, further comprising the step of tapping the slag from the furnace in a molten state, wherein the molten slag is sent to a zinc fumer/zinc fuming furnace.

19. A method according to claim 1, further comprising the step of tapping the matte from the furnace in a molten state, wherein the molten matte is sent to a copper smelter.

20. A method according to claim 1, wherein the slag contains between about 5 to 25 wt. % zinc, when the slag leaves the furnace.

21. (canceled)

22. A method according to claim 1, wherein the copper-containing matte substantially digests any incoming precious metals with the exception of the zinc which reports preferentially to the slag phase.

23. A method according to claim 1, wherein an inert slag or a glassy solid material comprising Fe, Ca, Al, and Si is formed as a by-product.

24. Zinc, when recovered from a zinc leach residue by a method according to claim 1.

25. Copper, when recovered from a sulfide material comprising copper and flux by a method according to claim 1.

26. One or more precious metals comprising silver and/or gold when extracted from a zinc leach residue by a method as defined according to claim 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0053] FIG. 1 shows a flowsheet of a process in accordance with one embodiment of the present invention;

[0054] FIG. 2 shows a pseudo-ternary phase diagram of SiO.sub.2FeO.sub.xZnO with fixed CaO content of 6 wt. % and fixed pO.sub.2 of 10.sup.8 atm. Yellow region denotes slag fully liquid at temperature of 1250 C. This diagram is reproduced from paper by: Liu, H., Cui, H., Chen, M., and Zhao, B. Phase Equilibria in the ZnO-FeO-SiO.sub.2CaO System at pO.sub.2 10.sup.8 atm, Calphad, v. 61 (2018) pp. 211-218;

[0055] FIG. 3 shows a diagram of slag liquidus temperatures, calculated for various mass ratios of CaO/SiO.sub.2, using the FTOxide database of FACTSAGE;

[0056] FIG. 4 shows a diagram of activity coefficient of ZnO (.sub.ZnO) and activity of ZnO (a.sub.ZnO), calculated for various mass ratios of CaO/SiO.sub.2, using the FTOxide database of FACTSAGE;

[0057] FIG. 5 shows a diagram showing the effect of slag temperature on partial pressure of Zn, calculated for various oxygen potentials, using the FTOxide database of FACTSAGE.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0058] The flowsheet shown in FIG. 1 is a flowsheet of one embodiment of the present invention. The flowsheet of FIG. 1 is a method for the pyrometallurgical processing of zinc leach residue mixed with sulfide copper materials that allows for good recovery of zinc and precious metals from the zinc leach residue. In the flowsheet shown in FIG. 1, zinc leach residues, sulfide copper materials, fluxes, and fuel, represented collectively by reference numeral 10, are fed to a feed preparation plant 12.

[0059] The mixed feed 14 is then supplied to a TSL furnace, in this case an ISASMELT top submerged lance furnace, 18. Air enriched with oxygen and, optionally, fuel 16 are fed to the TSL through the lance 16. The ISASMELT furnace 18 smelts the incoming feed materials in a turbulent bath to produce a molten slag and a molten copper matte. Most of the zinc in the feed materials reports to the slag and most of the copper, silver and gold in the feed materials report to the matte.

[0060] In a preferred embodiment, the ratio of oxygen-enriched air and fuel is controlled so that the oxygen potential or oxygen partial pressure of the furnace is maintained in the range of 10.sup.9.5 to 10.sup.7.5 atm, or from 10.sup.9 to 10.sup.8 atm. The copper matte that is formed readily digests most of the incoming precious metals but, as mentioned above, most of the zinc in the feed will report preferentially to the slag phase.

[0061] The ratio of feed materials and fluxes added to the ISASMELT furnace 18 is controlled to obtain a fluid slag that contains a generous amount of recoverable zinc. The slag will suitably contain between 5 to 25% zinc, and more suitably it will contain 10 to 20% zinc when it leaves the ISASMELT furnace 18. In the choice of slag composition and smelting temperature, processing conditions specific to preferred embodiments of this invention has been developed.

[0062] Experimentally determined slag liquidus temperatures at the relevant conditions are normally considered the best type of fundamental science relevant to smelter design. For example, FIG. 2 shows (in yellow) a tight confinement to the permissible slag composition consistent with a fully liquid slag containing ZnO, SiO.sub.2, FeO.sub.x and CaO in equilibrium with an atmosphere containing an oxygen partial pressure of 10.sup.8 atm. If reliant only on fundamental science, the choice of operating parameters would be made according to such a diagram.

[0063] However, it is a feature of this invention that the fundamental science published up until today is usually not sufficient to adequately define some nuances about the achievable parameters for smelting. In particular, two real-world phenomena appear to allow practical smelting operation to deviate noticeably from laboratory measurements and computational predictions.

[0064] Firstly, sulfur in the furnace, present in both the copper matte and the SO.sub.2-rich off-gas, partially invalidates published phase diagrams that have been calculated or experimentally derived from sulfur-free systems. With sulfur present in the furnace, slags with great fluidity are obtainable at temperatures up to 70 C. below the experimentally-determined liquidus.

[0065] Secondly, the ISASMELT furnace is able to operate reliably while producing slags at slightly sub-liquidus temperatures. Such slags might best be thought of as high-temperature slurries, with a small proportion of suspended solid particles inside a large fraction of liquid. Thus the ISASMELT furnace (item 18 of FIG. 1) can smelt a mixture of zinc residues and sulfide copper materials satisfactorily at a temperature below 1200 C. while producing a slag with a zinc content in the range of 10-20 wt. %, providing that the CaO/SiO.sub.2 ratio of the slag is maintained between 0.1 to 0.3 and the ratio of SiO.sub.2/(Fe+Zn) is between 0.6-0.8.

[0066] The off gases 20 from the ISASMELT furnace 18 are rich in sulfur dioxide (SO.sub.2), and desirably have a zinc fume content as low as possible. The generation of zinc fumes inside the ISASMELT furnace is unwelcome because it decreases the proportion of zinc reporting to the slag as ZnO, and therefore decreases the recovery of zinc through the subsequent slag fuming furnace (item 34 in FIG. 1).

[0067] It is well known to those skilled in the art, that furnaces tend to fume zinc out of slag very much faster when the concentration of CaO in the slag is increased, and when the slag temperature is increased. Compared to a conventional slag fuming furnace, where the CaO/SiO.sub.2 ratio in slag is typically around 0.7, there are two reasons why a lesser concentration of CaO is desirable in the slag of the ISASMELT furnace of embodiments of the current invention. Firstly, modest increases in the CaO/SiO.sub.2 ratio increase the liquidus temperature of the slag (as depicted in FIG. 3) and require the smelting temperature of the ISASMELT furnace to be increased in response. Secondly, the activity coefficient of ZnO in the slag (.sub.ZnO) becomes higher when the CaO/SiO.sub.2 ratio in slag is increased (as depicted in FIG. 4).

[0068] The increased .sub.ZnO causes the ZnO activity to be greater at any fixed concentration of ZnO in slag, and therefore more zinc will unhelpfully report to the fume during smelting in the ISASMELT furnace, according to such reactions as the following equation (2):

ZnO+CO.sub.(g).fwdarw.Zn.sub.(g)+CO.sub.2 (g)(2)

[0069] As can be seen in FIG. 5, having controlled the CaO/SiO.sub.2 ratio and other slag composition parameters carefully, the amount of zinc that reports to the gas phase is largely a function of furnace temperature, which in embodiments of this invention is also controlled below 1220 C. and preferably below 1200 C. The off gases 20 are sent to an acid plant 22 of conventional construction and operation where the off gases 20 are treated to form sulfuric acid 26 and an acid plant tail gas 24 from which the sulfur dioxide has largely been removed. The acid plant tail gas 24 may be vented to atmosphere through a flue or chimney, via methods well known to those skilled in the art.

[0070] Due to the turbulent nature of the molten bath in the ISASMELT furnace 18, the slag and the matte are mixed with each other. A mixture of the slag and matte 28 is removed from the ISASMELT furnace 18 and sent to a settling furnace 30. The settling furnace 30 is operated under relatively quiescent conditions and at a temperature that maintains the slag and the matte in molten state. The slag will separate from the matte, with the slag typically collecting on top of the matte in the settling furnace 30.

[0071] The zinc rich slag 32 is removed from the settling furnace 30 and sent to slag fuming furnace 34. Slag fuming furnace 34 is of conventional construction and operation and need not be described further. In the slag fuming furnace 34, zinc vaporises and is removed as a zinc fume 36. The zinc fume 36 comprises a gaseous stream containing vaporised zinc. Zinc can be recovered from the zinc fume 36 in accordance with known recovery processes. A copper speiss 38 and an inert slag 40 are also removed from the slag fuming furnace 34.

[0072] The copper speiss 38 and inert slag 40 may be removed in the molten state and allowed to solidify after removal from the furnace. The copper speiss may be sent for further treatment to recover copper therefrom. The inert slag 40 will form a glassy material when solidified. The inert slag 40 will contain Fe, Ca, Al, and Si compounds and any residual amounts of lead and arsenic will be chemically inert or bound within the inert slag, thereby rendering the inert slag suitable for disposal.

[0073] The copper matte 42 that is formed in the ISASMELT furnace 18 is sent to a conventional copper smelter 44. The ISASMELT furnace 18 is suitably operated such that partial combustion is achieved in the ISASMELT furnace 18 such that many of the gaseous components are substantially oxidised but leaving some uncombusted FeS, ZnS. Cu.sub.2S and PbS to form the molten matte in the bottom of the furnace. The composition of the matte depends upon how much uncombusted sulfide species are allowed to remain. The resulting matte will desirably contain between 40 to 75% copper which corresponds to an approximate partial pressure of oxygen, inside the ISASMELT furnace, between 10.sup.9 atm and 10.sup.8 atm.

[0074] As FIG. 5 depicts, the oxygen potential of the ISASMELT furnace does affect the partial pressure of Zn.sub.(g) in the furnace. As the matte grade is raised from 40 to 75% Cu, the oxygen potential rises from 10.sup.9 atm to 10.sup.8 atm, and the proportion of zinc reporting to fume according to reaction (2) is decreased. Other precious metals in the feed, such as gold and silver, also report to the matte phase. The matte 42 is suitable for addition to a normal copper smelter 44 for downstream processing. One product from the copper smelter 44 is recovered copper 46 with dissolved silver and gold.

[0075] Typical zinc leach residues that can form part of the feed to the ISASMELT furnace 18 have the following range of compositions (Table 1):

TABLE-US-00001 TABLE 1 Range of compositions of typical zinc leach residues fed to the ISASMELT furnace Species Zn Cu Fe S CaO SiO.sub.2 Ag Au Wt. % 10-25 0-5 10-25 3-9 0-3 2-10 0-0.1 0-0.01

[0076] Typical sulfide copper materials that form part of the feed to the ISASMELT furnace 18 have the following range of compositions (Table 2):

TABLE-US-00002 TABLE 2 Range of compositions of typical sulfide copper materials fed to the ISASMELT furnace Species Zn Cu Fe S CaO SiO.sub.2 Ag Au Wt. % 0-5 5-30 10-35 25-35 0-3 2-20 0-0.1 0-0.01

[0077] Owing to the minerals present in a zinc leach residue, which tend to be oxides and sulfates and their hydrated counterparts, its smelting tends to be endothermic, requiring a large energy input. Owing to the minerals present in the sulfide copper material, its smelting tends to be exothermic, and plentiful heat is generated. Coolant is sometimes required to maintain stable temperatures in a commercial Copper ISASMELT furnace. The combination of the two materials into a single smelting process is advantageous from this perspective.

Example

[0078] Industrial-scale tests were conducted at an operating ISASMELT facility employing a campaign of continuous furnace operation performed over two calendar days. The ISASMELT furnace used in the trial had a cylindrical vessel with a flat roof. The feed material was blended from a combination of zinc residues, sulfide copper concentrates and other smelter recycle streams.

[0079] The blended wet feed, along with silica/quartz flux and solid fuel was continuously transferred and discharged into the furnace by a series of conveyors. A central lance injected air, oxygen and trim fuel into the molten bath. The lance was sufficiently immersed into the molten bath so that the injected air, oxygen, and trim fuel created in a high level of agitation of the liquid, ensuring a rapid reaction between the raw materials and the oxygenated slag bath.

[0080] The average composition of the blended feed is shown in Table 3 and the average key furnace parameters from the trial are shown in Table 4.

TABLE-US-00003 TABLE 3 Average concentrate composition from Plant Trial Element Cu Pb Zn Fe S SiO.sub.2 CaO MgO Al.sub.2O.sub.3 Unit Wt. % Wt. % Wt. % Wt. % Wt. % Wt. % Wt. % Wt. % Wt. % Assay 10.84 5.30 10.01 20.80 21.16 6.20 1.33 0.58 4.59

TABLE-US-00004 TABLE 4 ISASMELT furnace parameters Parameter Average Value Blended concentrate rate (dmt/h) 39.0 Solid fuel rate (dmt/h) 2.96 Silica/quartz rate (dmt/h) 4.21 Lance process air (Nm.sup.3/h) 6400 Lance process oxygen (Nm.sup.3/h) 7120

Test Results

[0081] The industrial trial confirmed that the feed materials were successfully incorporated into the bath and formed separate matte and slag phases. Removal of the molten products was achieved by opening a single taphole at the base of the furnace and settling of the materials was performed in the site 3-in-line electric furnace.

[0082] During the trial process off-gases passed through a waste heat boiler and an electrostatic precipitator for removal of dust prior to being directed to the site acid plant for desulfurisation. The operation of the off-gas handling system was successfully able to achieve the targets at each system interface, most notably the exit temperature of the waste heat boiler was able to be kept between 340-360 C. and the SO.sub.2 concentration in the off-gas to the acid plant was maintained between 11-13 vol. %.

[0083] During the industrial trial samples were taken using cast iron spoons for the molten phases, whilst traditional sampling methods were used for the other solid streams. Samples were ground and analysed by X-ray fluorescence (XRF).

[0084] The production rates and compositions of key phases are shown in Table 5 and Table 6, respectively.

[0085] The assays and mass flows confirmed the majority of the zinc, over 80%, in the feed materials reported to the slag phase in the furnace. The data confirmed that 95% of the copper, and hence by association silver and gold in the feed materials, reported to the matte phase. Fluid slags were attained during the industrial campaign with the actual CaO/SiO.sub.2 ratio of the slag maintained at 0.22 (target between 0.1 to 0.3) and the actual SiO.sub.2/(Fe+Zn) ratio of the slag maintained at 0.63 (target between 0.6-0.8).

TABLE-US-00005 TABLE 5 ISASMELT furnace parameters Parameter Value Settled Slag (t slag/t concentrate) 0.604 Settled Matte (t matte/t concentrate) 0.191 Combined Dust (t dust/t concentrate) 0.060

TABLE-US-00006 TABLE 6 ISASMELT furnace parameters Parameter Cu Zn Fe S SiO.sub.2 CaO Units Wt. % Settled slag 0.8 14.0 30.8 0.4 28.4 6.3 Settled matte 54.0 3.3 7.6 16.2 0.6 Combined dust 1.1 18.7

[0086] The ISASMELT furnace bath temperature was measured continuously during the trial, averaging 1175 C. (target less than 1200 C.), using a thermocouple placed through the refractory lining of the furnace. Confirmation of the bath temperature was obtained using a disposable dip-tip measurement during tapping.

[0087] The molten matte was treated in the broader copper smelter flowsheet and furnace of the site and successfully converted to blister copper and fire refined to anode copper prior to electrolytic refining. The precious metals slimes was recovered after the copper refining was completed and put through a standard process for the production of separate precious metals streams. The molten slag was treated at the site for zinc recovery. This confirmed the process flowsheet shown in FIG. 1.

[0088] The process shown in FIG. 1 allows the following desirable outcomes to be achieved: [0089] a) Obtain high recoveries of one or more valuable elements selected from Cu, Ag, Au and Zn. [0090] b) Create an SO.sub.2-rich gas that is suitable for feeding to a conventional metallurgical acid plant. [0091] c) Create a glassy solid slag material for sale or disposal of the elements Fe, Ca, Al and Si in which any residual amounts of Pb and As will be chemically inert.

INDUSTRIAL APPLICABILITY

[0092] The invention as described above relates to a method for treating zinc leach residue. Zinc leach residue, produced from traditional zinc hydrometallurgy processes, is not only a hazardous waste but also a potential valuable solid. The invention as described thereby demonstrates industrial applicability in the economic value of the zinc recovered, and environmental efficacy by remediating an otherwise hazardous waste material.

[0093] Furthermore, the inventive method provides means for recovering precious metals such as silver and gold from zinc residues. Such precious metals had often gone unrecovered due to process efficiencies and practical difficulties in extracting them from the zinc residue. In the present invention, such precious metals report to the copper-containing matte, from which they may be extracted or on-sold either as the matte, per se, or by converting it to copper cathode and anode slimes.