Abstract:
A method of recovering at least one base metal from a concentrate wherein a residue of a primary bioleach of the concentrate, under mesophilic and moderate themophilic conditions, is processed to recover at least one metal, and the base metal is recovered from a solution, produced by a secondary bioleach under thermophilic conditions, of a residue of the metal recovery process.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to the treatment of a base metal containing concentrate.  
         [0002]     The recovery of a base metal such as copper from a concentrate, which contains arsenic, using a bioleaching process, can be problematic for it is necessary to avoid producing a bioleach residue which is contaminated with arsenic.  
         [0003]     U.S. Pat. No. 6,461,577 addresses the problem of arsenic toxicity of extremely thermophilic bacterial cultures by means of a two-stage leaching process. In a first mesophilic stage a major part of the arsenic contained in the material being treated is leached from the material and then oxidised from As(III) to As(V). The remaining leachable metal content of the material being treated is leached out in a second thermophilic stage. The concentration of pentavalent arsenic falls quickly and the toxic effect thereof on the thermophilic bacteria thus falls at the same rate.  
         [0004]     It is desirable though to remove the soluble arsenic as ferric arsenate which passes EPA limits and is safe for land disposal and which, to the extent possible, is not in a bioleach residue.  
       SUMMARY OF INVENTION  
       [0005]     The invention provides a method of treating a concentrate containing at least one base metal which includes the steps of subjecting the concentrate to a primary mesophilic and moderate thermophilic bioleaching process to leach sulphides in the concentrate, processing a residue of the primary bioleach process to recover at least one metal from the primary bioleach residue, subjecting a residue from the metal recovery process to a thermophilic secondary bioleaching process to release the at least one base metal from the metal recovery residue into solution, and recovering the at least one base metal at least from the solution produced by the thermophilic secondary bioleaching process.  
         [0006]     The primary bioleaching process may be carried out at a temperature of from 35° C. to 50° C.  
         [0007]     Preferably the at least one base metal is also recovered from a solution produced by the primary bioleach bioleaching process.  
         [0008]     The method preferably includes the step of preleaching the concentrate, before the primary bioleach bioleaching process, using leach solution from at least one of the bioleaching processes. Preferably the leach solution is derived from the primary bioleaching process and the thermophilic secondary bioleaching process.  
         [0009]     The preleaching step is used to remove easily leachable base metal from the concentrate before the primary bioleaching process. Elemental sulphur which may accumulate during the preleach step due to rapid leaching of easily leachable sulphides, may be removed during the bioleaching stages, especially during the thermophilic secondary bioleaching process at elevated temperatures.  
         [0010]     The primary bioleaching process may be carried out in a series of continuously stirred tank reactors which are operated at a temperature of from 35° C. to 50° C. in the presence of an active mixed culture of mesophilic and moderate thermophilic microorganisms.  
         [0011]     A mixed culture of mesophile (20° C. to 40° C.) and moderate thermophile (40° C. to 55° C.) microorganisms is preferably used to maximise sulphide bioleaching and sulphur biooxidation during the treatment process. The mixed culture may contain microorganisms like  Leptospirillum ferrooxidans  and  Acidithiobacillus caldus , a good iron oxidiser and a good sulphur oxidiser respectively.  
         [0012]     The primary bioleach process may also contain thermophilic microorganisms, which are not effectively active at the temperature range of 35° C. to 50° C. Such microorganisms will, however, still be living but will be dormant or slowly metabolising. When the temperature increases during the thermophilic secondary bioleaching process these microorganisms will reactivate their activity. This may be very useful for base metal concentrates, as the thermophilic secondary bioleaching stage would be continuously re-inoculated.  
         [0013]     The pH of the concentrate or pulp in reactors in which the primary bioleaching is carried out may be controlled at a value of from 1,2 to 1,7. This may be achieved by the addition of limestone or raffinate produced in the base metal recovery step, to the reactors.  
         [0014]     Oxygen may be supplied to the concentrate in the reactors in the form of enriched air which may contain from 95% to 98% oxygen, during at least part of the bioleaching processes.  
         [0015]     An objective of operating the primary bioleaching process under the aforementioned conditions is to maximise the leaching of the sulphides in the concentrate and to maximise mass loss, and to minimise the precipitation in pentavalent form of arsenic which may be present in solution. The product from the primary bioleach residue may contain high concentrations of elemental sulphur due to the maximised bioleaching conditions.  
         [0016]     In the metal recovery process toxic silver may be removed from the primary bioleaching residue. The silver may be removed using a brine leaching process.  
         [0017]     The thermophilic secondary bioleaching process may be carried out in a series of continuously stirred tank reactors at a temperature of from 65° C. to 80° C. in the presence of active quantities of extreme thermophilic microorganisms.  
         [0018]     The method may include the step of controlling the pH of the pulp in the thermophilic reactors at a value of from 1,0 to 1,7. This may be achieved by the addition of limestone or raffinate produced in the metal recovery step. Oxygen &amp; carbon dioxide may be supplied to the reactors in the form of enriched gas containing from 95% to 98% oxygen and 1% to 5% carbon dioxide.  
         [0019]     An objective of operating the thermophilic secondary bioleaching process under the aforementioned parameters is to maximise the oxidation of sulphides minerals and mass loss, and to minimise the precipitation in pentavalent form of arsenic which may be present in solution.  
         [0020]     Furthermore sulphur oxidation at thermophilic temperature conditions is maximised and thus any elemental sulphur produced during the proceeding preleach and primary bioleach may be fully oxidised. This is very important if further treatment of the thermophilic secondary bioleach residue is required for precious group metals (PGM&#39;s) recovery like gold. Elemental sulphur increases cyanide consumption during cyanidation to recover gold and thus contributes significantly to the increase in cyanidation costs. Additionally, elemental sulphur not oxidised decreases the acid produced in the bioleach solution and thus may decrease the effectiveness of any preleach step using recycled bioleach solution.  
         [0021]     As indicated the at least one base metal is recovered from the leach solutions produced by the bioleaching processes. Preferably the pH of the solution produced in the preleaching step is adjusted to maximise recovery of the at least one base metal using solvent extraction techniques.  
         [0022]     Arsenic present in the solution may be caused to precipitate as ferric arsenate by increasing the pH of the solution to at least 2.  
         [0023]     The pH of the solution may be increased by the addition of limestone slurry to the solution.  
         [0024]     The pH adjustment may be carried out in a series of continuously stirred tank reactors which are operated at a temperature of from 60° C. to 80° C.  
         [0025]     The at least one base metal, eg. copper, may be recovered by concentrating the copper and stripping, followed by cathode production by electrowinning.  
       BRIEF DESCRIPTION OF THE DRAWING  
       [0026]     The invention is further described by way of example with reference to the accompanying drawing which is a flow sheet illustrating various steps in a method of treating a concentrate obtaining at least one base metal in accordance with the principles of the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0027]     In the method of the invention a concentrate  10  which contains a base metal such as copper and which may have a high arsenic content is subjected to a preleaching step  12 . In this step the fresh concentrate is contacted with bioleach overflow solutions  14  and  16  respectively produced in subsequent primary and thermophilic secondary bioleaching stages  18  and  20 .  
         [0028]     The solutions  14  and  16  are rich in ferric and remove easily leachable copper from the feed  10 . This ensures a lower residual copper tenor in the bioleaching tanks in the stages  18  and  20 .  
         [0029]     The product  22  of the preleaching stage is subjected to solid/liquid separation  24 . An overflow solution  26  from the separation step  24  is directed to a pH adjustment stage  28  while the underflow  30 , diluted with water  32  and raffinate  34  from a solvent extraction section  36 , is fed to the primary bioleaching stage  18 .  
         [0030]     The purpose of the primary bioleaching stage  18  is to oxidise sulphide minerals in the feed and release base metals of interest into solution. The bioleaching is carried out in a series of continuously stirred tank reactors which are operated at a temperature of 35° C. to 50° C. in the presence of active quantities of mesophilic and moderate thermophilic microorganisms.  
         [0031]     The pH of the pulp of the reactors in the primary bioleaching stage is controlled at a value of from 1,2 to 1,7 by the addition of limestone  40  or raffinate  34 . Oxygen  42 , required for the oxidative reaction, is supplied in the form of enriched air with an oxygen content of from 95% to 98%.  
         [0032]     By operating the primary bioleaching stage  18  under the aforementioned conditions the oxidation of the sulphide minerals and the mass loss are maximised while, if arsenic is present in the feed, the precipitation thereof in pentavalent form is minimised.  
         [0033]     The product  44  of the primary bioleaching section  18  reports to bioleach thickening and washing  46 . As has been indicated the overflow solution  14  is fed to the preleaching step  12  while the underflow  48  is the feed to a metal recovery section  50 .  
         [0034]     The purpose of the step  46  is to separate the liquids and the solids so that the base metals of interest and arsenic, if present, report to the pH adjustment section  28  via the preleaching step  12 . In the metal recovery step  50  toxic silver  52  is removed from the primary bioleaching residue  48  using a brine leaching or other suitable method.  
         [0035]     The residue  54  from the metal recovery step is repulped with water  56  and raffinate  34  and the resulting slurry is fed to the thermophilic secondary bioleaching stage  20 .  
         [0036]     The purpose of the stage  20  is to oxidise, to the extent possible, the sulphide minerals and the elemental sulphur which were not leached in the primary bioleaching stage  18 . The base metals of interest are thereby released into solution. The thermophilic secondary bioleaching process is carried out in a series of continuously stirred tank reactors which are operated at a temperature of from 65° C. to 80° C. in the presence of active quantities of extreme thermophilic microorganisms.  
         [0037]     The pH of the pulp in the thermophilic reactors is controlled at a value of from 1,0 to 1,7 by the addition of limestone  40  or raffinate  34 . Oxygen  42  required for the oxidative reactions is supplied in the form of enriched gas with an oxygen content of from 95% to 98%. Carbon dioxide  57  may be required for improved thermophilic cell growth is supplied in the form of enriched gas with a carbon dioxide content of 1% to 5% by volume. By operating the thermophilic secondary bioleaching sections under these conditions the oxidation of the sulphide minerals and the mass loss are maximised while the precipitation of arsenic which may be present in the slurry 54, in the form of pentavalent arsenic, is minimised.  
         [0038]     The product  60  of the thermophilic bioleaching section  20  reports to a bioleach thickening and washing step  62 . The overflow solution  16  is fed to the preleaching section  12  while the underflow  64  is directed to a tailings pond  66  for disposal. If the underflow  64  contains PGM&#39;s then the underflow is directed to a metal recovery step  67  where the metal is removed from the underflow using cyanide as a leaching process for gold or other suitable method.  
         [0039]     The purpose of the step  62  is to separate liquid and solids so that base metals of interest and arsenic, if present, are reported in solution to the pH adjustment section  28  via the preleaching stage  12 .  
         [0040]     The pH adjustment section  28  includes a series of continuously stirred tank reactors which are operated at a temperature of from 60° C. to 80° C. The pH of the solution  26  is increased to a required level using limestone  40  or any other suitable neutralising agent. The product  70  of the pH adjustment section is then thickened in a step  72 . The thickener underflow  74 , which contains precipitated ferric arsenate, is directed to a tailings pond  76  for disposal. The overflow from the thickener step reports as pregnant leach solution (PLS)  80  to the solvent extraction section  36 .  
         [0041]     The purpose of the pH adjustment section  28  is to increase the pH of the pregnant leach solution, which is fed to the solvent extraction section  36 , to above 2,0 so that the solvent extraction efficiency is maximised. Arsenic which is present in the solution  26  is caused, by the increase in the pH, to precipitate primarily as ferric arsenate which is not readily dissolved. The ferric arsenate passes EPA limits and is safe for land disposal.  
         [0042]     In the solvent extraction section  36  dissolved copper is recovered from the pregnant leached solution. The copper is stripped followed by cathode production ( 84 ) by electrowinning.  
         [0043]     In the process of the invention the base metal containing concentrate is subjected to primary mesophilic and or moderate thermophilic leaching, metal recovery and thermophilic secondary leaching in combination so that secondary sulphides are successfully and economically leached in the primary section, toxic silver is removed in the metal recovery section, and a residue containing unleached primary sulphides and elemental sulphur is leached to completion successfully and economically in the thermophile secondary section. If arsenic is present in the concentrate the primary and thermophilic secondary sections are operated so that the redox potential of the solutions produced result in the natural oxidation of As(III) to As(V). Arsenic precipitation in the bioleaching sections is intentionally minimised so that the arsenic is precipitated externally in the pH adjustment section  28 . This avoids the production of a bioleach residue contaminated with arsenic.  
         [0044]     It is cost effective to reduce the arsenic reporting to the thermophilic stage  20  by causing the arsenic to precipitate in a separate dedicated process step ie. the pH adjustment section  28 . By minimising precipitation in the mesophilic stage  18  the mass loss throughout the process is maximised. This reduces the capital and operating cost of the downstream processes including the thermophilic section  20 .  
         [0045]     Furthermore sulphur oxidation at thermophilic temperature conditions is maximised and thus any elemental sulphur produced during the proceeding preleach and primary bioleach may be fully oxidised. This is important if further treatment of the thermophilic secondary bioleach residue is required for precious group metals (PGM&#39;s) recovery like gold. Elemental sulphur increases cyanide consumption during cyanidation to recover gold and thus contributes significantly to the increase in cyanidation costs. Additionally, elemental sulphur not oxidised decreases the acid produced in the bioleach solution and thus may decrease the effectiveness of any preleach step using recycled bioleach solution.