Abstract:
The invention relates to a process for the pre-treatment of feed to a non-stirred surface heap leach bioreactor by applying in sequence first and second pre-treatment solutions to a feed to a non-stirred surface heap leach bioreactor, in which the first solution has an iron content greater than 5 g/l and pH below 2, and the second solution contains a substantially higher microbial population. The invention further relates to methods of adapting a microbial population for use in a non-stirred surface heap leach bioreactor.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a process of pre-treatment of feeds to non-stirred surface bioreactors and a method of adapting a microbial population for use in a non-stirred surface heap leach bioreactor. 
       BACKGROUND TO THE INVENTION 
       [0002]    A method of treating metal bearing solids using a non-stirred surface bioreactor is described in U.S. Pat. No. 5,766,930, which is incorporated herein by reference. Such a method uses what is hereinafter termed a “surface bioreactor”. 
         [0003]    Some metal bearing solids, usually flotation concentrates, do not respond favourably or quickly enough to bio-processing and poor bacterial activity, evidenced by low redox potential of the liquid phase, is normally a symptom of such problems. 
         [0004]    During the adaptation of micro-organisms to specific concentrates, it has been observed that in some instances the micro-organisms either adapt very slowly, and in rare cases not at all, to the specific concentrate. This indicates that the leaching microbes are being subject to some toxic effect from compounds arising from the concentrate. This results in an extremely long lag phase before the microbe population can increase in number to high enough levels to oxidize the sulphides. This extends the leaching time of the process which reduces the economic benefits of the process. 
         [0005]    The operation of a surface bioreactor typically includes the coating of a flotation concentrate onto a substrate (typically crushed and sized rock), stacking the coated rock into a heap and inoculating the heap with an inoculum, typically via the irrigation of recycled heap effluent. Inoculation by this method is inefficient due to the natural stickiness of microbes giving rise to low penetration rates of microbes through the heap and the low solution application rates used, typically around 20 l/m 2 /hour. 
         [0006]    After the sulphides have been oxidised, the oxidized portion of the heap is taken down and the oxidised concentrate is washed off the rock. The rock is frequently recycled and coated with fresh concentrate. The oxidised concentrate is then thickened and, for refractory gold operations, the thickened oxidised concentrate being processed using cyanidation to recover the gold. Such a process is frequently tested in columns in the laboratory. 
         [0007]    Where the aforementioned problems with apparent toxicity and slow distribution of microbes through the coated rock are encountered, the redox potential in the effluent solution only increases slowly, delaying the onset of oxidation and increasing the overall leaching time. Leaching time is an important consideration in surface bioreactor plants as increased leaching time gives rise to higher pad area requirements and increased metal inventory within the pad. In a typical refractory gold project this would yield a substantial cost benefit in terms of lower inventory and reduced capital cost. 
       OBJECT OF THE INVENTION 
       [0008]    It is an object of the invention to provide a pre-treatment solution for feeds to a non-stirred surface bioreactor and a method of pre-treating feeds to a non-stirred surface bioreactor which at least partly overcomes the abovementioned problems. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with this invention there is provided a process for the pre-treatment of a feed to a non-stirred surface heap leach bioreactor which includes the step of applying in sequence first and second pre-treatment solutions to a feed to a non-stirred surface heap leach bioreactor, in which the first solution has an iron content greater than 5 g/l and pH below 2, and the second solution contains a substantially higher microbial population. 
         [0010]    There is further provided the microbial population of the second solution to be selectively adapted relative to a heap PLS solution. 
         [0011]    There is also provided for the process to include the addition of additional sulphuric acid concurrently with the iron rich first solution. 
         [0012]    There is further provided for the second solution to comprise a solution from a dewatering step of oxidised product from the non-stirred surface bioreactor, and for the iron rich first solution to comprise a re-circulating irrigation solution from a non-stirred surface bioreactor process. 
         [0013]    There is still further for the microbial population of the second solution to have been enriched and adapted in an inoculum generator which has been charged with concentrate, sulphur or other material to selectively increase the population of specific microbes in the solution. 
         [0014]    There is also provided for the microbial population of the second solution to contain microbes from an inoculum generator of which the temperature is operated in a predeterminable temperature range to increase the population of specific microbes in the solution. 
         [0015]    There is further provided for the solution with the adapted microbial population content to contain microbes from an inoculum generator which is fed with concentrate to select microbes most suitable for specific toxic compounds contained in the concentrate. 
         [0016]    There is still further provided the solution with the adapted microbial population content to contain microbes from an inoculum generator which is fed with a sulphur species to select microbes most suitable for sulphur oxidation. 
         [0017]    There is also provided for at least part of the solid phase to be separated from the liquid phase after pre-treatment, either prior to treatment with another solution or prior to coating and stacking onto a non-stirred surface bioreactor. 
         [0018]    The invention further provides for a method of adapting a microbial population for use in a non-stirred surface heap leach bioreactor which includes a microbe selection step in which a solution which contains the microbes is passed through an inoculum generator which is fed with concentrate to select microbes most adapted for specific toxic compounds contained in the concentrate in the non-stirred surface heap leach bioreactor, an inoculum generator which is fed with a sulphur species to select microbes most suitable for sulphur oxidation in the non-stirred surface heap leach bioreactor, further alternatively an inoculum generator which is operated in a specific temperature range to selectively favour a specific microbe most suitable for the temperature operating regime used in the bioreactor in the non-stirred surface heap leach bioreactor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Embodiments of the invention are shown in the accompanying drawings in which: 
           [0020]      FIG. 1  shows an embodiment of the invention for processing of a toxic gold bearing arsenopyritic concentrate, containing substantial carbonate mineralization and a tendency to produce elemental sulphur during the bio-oxidation process; and 
           [0021]      FIG. 2  shows another embodiment of the invention for processing a pyritic gold bearing concentrate with minor carbonate and stibnite that shows minor toxicity in laboratory amenability tests. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Two factors are important regarding the required leaching time for a particular concentrate in a surface bioreactor. 
         [0023]    Firstly, some concentrates appear to give rise to toxicity effects which leads to an extended lag phase with the bioleaching microbes. The reasons for this behaviour are not understood entirely, but it may be due to organic compounds which are present in the concentrate, for example flotation reagents. Another reason may be the presence of a relatively fast leaching mineral species that releases toxic compounds into the solution which inhibits the action of the micro-organisms. In either case, removal in part or whole of the organic and/or reaction products from the fast leaching species, prior to stacking on the heap, would be beneficial. Toxicity is evidenced by a poor redox potential persisting in effluent solutions for several weeks until the bacterial population becomes well established, whereupon the redox rises to &gt;600 mV. 
         [0024]    Secondly, it is well known that bioleaching microbes excrete exopolymers and are “sticky” in their natural state. Thus inoculation of a surface bioreactor using irrigation solution results in slow penetration of bacteria in the heap, also increasing period before leaching begins efficiently. The natural growth of the leaching microbes that have penetrated the heap will be further inhibited by any toxic compounds coming into the solution phase. 
         [0025]    Flotation concentrates processed using surface bioreactors are usually coated, as thickened slurry from a flotation plant, onto the rock. Alternatively the concentrate may be filtered or dried, re-pulped with water, and then coated onto the rock. The density of the concentrate pulp is an important factor in maintaining adherence of the concentrate to the rock. The concentrate could be re-pulped to the correct density with inoculum (for example re-circulating PLS solution), however any toxic compounds would be stacked along with the concentrate. 
         [0026]    In another mode for concentrates containing toxic compounds, improved bacterial activity in the stacked heaps can be achieved by pre-treating the flotation concentrate with an acidic solution of ferric sulphate containing substantial quantities of leaching microbes, in one or more pre-treatment reactors. 
         [0027]    Large volumes of solution can be rapidly mixed with the incoming concentrate in a pre-treatment reactor. To apply a similar volume of solution via the irrigation system would be much slower. For example, to apply 10 m 3  of solution to a tonne of concentrate stacked in a heap with rock would take about 27 days using an irrigation system, assuming an irrigation rate of 20 l/m 2 /hour and 1.3 t concentrate stacked per m 2 . In contrast, 10 m 3  of solution per hour could be contacted with one tonne of solids every hour in a pre-treatment reactor. 
         [0028]    The ability to rapidly mix large volumes of solution with the concentrate has five important direct and indirect benefits. 
         [0029]    Firstly, the quantity of leaching microbes available for leaching is improved in situ as the coated rock is stacked. If the heap is inoculated using irrigation solution and that solution contains 1×10 6  microbes per ml, some 7.7×10 6  microbes would be applied per g of concentrate in the 27 day period. Whilst the microbial population would normally naturally increase, such increase in population will be substantially inhibited in the presence of any toxic compounds present in the solution. Also given the low solution application rates in heaps, such effects are likely to persist with time. Using the pre-treatment reactor about 1×10 7  microbes can contacted with a gram of concentrate every hour. Again though, any toxic compounds will remain in solution. 
         [0030]    Secondly, large volumes of ferric solution provide an environment where fast leaching mineral phases can be leached, especially those producing potentially toxic products such as As 3+  from the leaching of arsenopyrite, albeit partially. Using irrigation solution at, say 15 g/l Fe 3+ , at a rate of 20 l/m 2 /hour onto a heap containing 1.3 t/m 2  of concentrate, applies ferric at a rate of 231 g per hour per tonne of concentrate. Consider a concentrate containing 5% of a fast leaching compound requiring 1:1 ferric addition on a mass basis i.e. the 50 kg of fast leaching species per tonne concentrate requires 50 kg of ferric. It would take 216 hours to apply the ferric using the irrigation solution. Using a pre-treatment reactor at 10 m 3  per tonne concentrate with similar ferric concentrations, ferric is applied at a rate of 150,000 g per hour per tonne concentrate, three times the amount required to leach our hypothetical fast leaching species. 
         [0031]    A third and indirect benefit is that mixing of a large volume of solution with the concentrate requires that the resulting product be dewatered using a thickener and/or filter. Such a process step provides an opportunity to bring the concentrate from the flotation plant consistently to the correct pulp density, which is a very important factor in maintaining adherence of the concentrate to the rock. 
         [0032]    Fourthly, any toxic compounds are diluted substantially and, coupled with the dewatering step above, a large proportion of any soluble toxic compounds associated with the liquid phase may be removed and immediately discarded. Additionally the concentration of toxic compounds in the liquid phase of the coating on the rock is reduced directly prior to stacking. 
         [0033]    The fifth benefit is that many flotation concentrates contain acid consuming carbonate minerals. These carbonates may be removed with sulphuric acid and concentrated sulphuric acid is usually used for this. However the direct addition of sulphuric acid to a carbonate containing concentrate usually results in severe foaming and expansion of the slurry, which presents practical difficulties. However the addition of large solution volumes enables concentrated sulphuric acid to be mixed into the slurry without concerns of foaming. Also the acid generated in the PLS by bioleaching pyrite in the concentrate can be put to use. Additionally, because the carbonate is removed upfront, the pH in the PLS drops quickly to that suitable for bioleaching, reducing the required time on the leach pad. For example to add 100 kg/t of acid to a concentrate via irrigation solution containing 5 g/l sulphuric acid at a rate of 20 l/m2/hour would take 1000 hours, but a similar amount of acid can be applied using PLS solution in a matter of hours, especially with a top-up of fresh acid. 
         [0034]    The acidic solution of ferric sulphate containing substantial quantities of leaching microbes may be tailored for specific applications by judicious use of available plant solutions. The inventors have observed that the re-circulating irrigation solution usually has quite a high population of microbes typically around 1×10 6  per ml and is enriched in iron, typically 5-40 g/l Fe with a pH of about 1.5. However the overflow from the reclaim thickener (that dewaters the oxidised concentrate) typically contains about an order-of-magnitude higher content of microbes at around 1×10 7  per ml, but is normally low in iron content (the heap having been rinsed with water prior to being taken down) typically &lt;2 g/l Fe and higher pH at around pH3. Additionally, microbes from the reclaim thickener overflow have been washed off the heap mass and are likely more adapted to conditions within the heap, whereas those in the irrigation solution may not. Thus using various mixtures of the reclaim thickener overflow and re-circulating irrigation solution in the pre-treatment steps, the type and quantity of leaching microbes and iron content of the solution phase in the pre-treatment steps may be tuned for a specific concentrate. Additional sulphuric acid may be added as appropriate to dissolve carbonate minerals. 
         [0035]    Additionally some or all of the irrigation solution and/or the reclaim thickener overflow solution may have their microbe content enriched by a microbe selection step. By passing the solutions through an inoculum generator, fed with small amounts of concentrate to select microbes most adapted to any toxic compounds contained in the concentrate. The inoculum generator may be fed using other materials (for example sulphur) to select the population of a particular microbial species that has the desired genetic trait for example sulphur metabolism. Alternatively it may be operated in a specific temperature range, to increase the population of a desired microbe (for example extreme thermophiles, by operating at &gt;60 Deg C.). It may also be possible to use a combination of feeding small amounts of concentrate to the inoculum generator and operating it in specific temperature range to select the most suitable microbe. 
         [0036]    Whilst the cost of pre-treatment reactors, thickeners and inoculum generators will not be insignificant, the savings due to a reduction in leaching period will typically be much higher. 
         [0037]      FIG. 1  shows a surface bioreactor flowsheet for the treatment of the gold bearing arsenopyrite concentrate showing severe toxicity in laboratory tests, with major carbonate content. The oxidised concentrate has high levels of elemental sulphur. 
         [0038]    A bleed stream ( 120   a ) from the irrigation solution pond ( 120 ) is mixed with the incoming flotation concentrate ( 1 ) in a first pre-treatment reactor ( 30 ), to which 100 kg/t of sulphuric acid ( 31 ) is added. The leached solids ( 30   b ) are fed to a pre-treatment thickener ( 40 ). The pre-treatment thickener overflow ( 40   a ) goes to neutralisation ( 60 ) and disposal. The pre-treatment thickener underflow ( 40   b ) is fed to a second pre-treatment reactor ( 50 ), where a solution ( 100   d ) from an inoculum generator ( 130 ) fed with sulphur ( 130   a ) and a bleed ( 100   b ) of the oxidized concentrate thickener overflow ( 100   a ) is added. The second pre-treatment reactor product ( 50   b ) goes to a coating device ( 70 ) along with recycled support rock ( 200 ). The coated support rock ( 70   a ) is fed to the surface bioreactor heap ( 80 ). 
         [0039]    The surface bioreactor heap ( 80 ) is continuously irrigated with solution ( 120   a ) derived from the irrigation solution pond ( 120 ). The heap effluent solution ( 80   a ) flows back to the irrigation solution pond ( 120 ). Low pressure air ( 117   a ) is blown through the heap surface bioreactor ( 80 ). 
         [0040]    Once oxidation is completed the oxidised portion ( 80   b ) of the surface bioreactor heap ( 80 ) is removed and fed into an oxidized concentrate screen ( 90 ). The washed support rock ( 200 ) is fed to the coating device ( 70 ). The oxidized fines ( 90   b ) are fed to an oxidized concentrate thickener ( 100 ) from which the thickener underflow ( 100   d ) is further processed using cyanidation to recover the gold. The oxidized concentrate thickener overflow ( 100   a ) is split into a portion ( 100   c ) going to the solution pond ( 120 ) and a bleed portion ( 100   b ) going to an inoculum generator ( 130 ) fed with elemental sulphur ( 130   a ). 
         [0041]      FIG. 2  shows a surface bioreactor flowsheet for the treatment of the gold bearing pyrite concentrate, with minor carbonate and stibnite content. 
         [0042]    A bleed stream ( 25   a ) from the irrigation solution pond ( 25 ) is mixed with the incoming flotation concentrate ( 21 ) in a first pre-treatment reactor ( 22 ), to which some sulphuric acid ( 23 ) is added. The leached solids ( 26 ) are fed to a second pre-treatment reactor ( 26   a ), where a bleed ( 27 ) of the oxidized concentrate thickener overflow ( 28   a ) is added. The second pre-treatment reactor product ( 29 ) is thickened in a pre-treatment thickener ( 29   a ). The pre-treatment thickener overflow ( 29   b ) goes to neutralisation ( 11 ) and disposal. The pre-treatment thickener underflow ( 12 ) is fed to a coating device ( 13 ) along with recycled support rock ( 20 ). The coated support rock ( 14 ) is fed to the surface bioreactor heap ( 15 ). 
         [0043]    The surface bioreactor heap ( 15 ) is continuously irrigated with solution ( 17 ) derived from the irrigation solution pond ( 25 ). The heap effluent solution ( 16 ) flows back to the irrigation solution pond ( 25 ). Low pressure air ( 17   a ) is blown through the heap surface bioreactor ( 15 ). 
         [0044]    Once oxidation is completed the oxidised portion ( 18 ) of the surface bioreactor heap ( 15 ) is removed and fed into an oxidized concentrate screen ( 19 ). The washed support rock ( 20 ) is fed to the coating device ( 13 ). The oxidized fines ( 21 ) are fed to an oxidized concentrate thickener ( 28 ) from which the thickener underflow ( 28   b ) is further processed using cyanidation to recover the gold. The oxidized concentrate thickener overflow ( 28   a ) is split into a portion ( 22   a ) going to the solution pond ( 25 ) and a bleed portion ( 27 ) going to the second pre-treatment reactor ( 26   a ). 
         [0045]    It will be appreciated that the embodiments described above has been included by way of example only, and is not intended to limit the scope of the invention. It is possible to alter certain aspects of the embodiment within the scope of the invention. 
         [0046]    It is, for example, possible that the invention can be used for the processing of base metal concentrates, as well as in all cases where ore may be used as the substrate. It is also possible to use the invention in stirred tank processing of gold and base metal concentrates. 
         [0047]    It is also possible to include additional a thickener in the process before the coating step, which may be required to increase the solids content of the material to be coated on the feed to the bioreactor. Referring to  FIG. 1 , such a thickener could be located between Pre-treatment Reactor 2 ( 50 ) and the Coating step ( 70 ), in other words in stream  50   b.