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Semi-continuous biooxidation of the Chongyang refractory gold ore
Mittemls
PII:SO892-6875(97)00037-X
Engineering, Vol. 10, No. 6, pp. 517-583, 1997 8 1997 Published by Else&r Science Ltd Printed in Great Britain. All rights resewed 0892-6875/97 SI7.00+0.00
SEMI-CONTINUOUS BIOOXIDATION OF THE CHONGYANG REFRACTORY GOLD ORE
Y. WEI§, K. ZHONGS, E.V. ADAMOVT and R.W. SMITH* 0
Wuhan Institute of Chemical Technology, 430074, P. R. China t Moscow Institute of Steel and Alloy, 117936, Moscow, Russia $ University of Nevada, Reno, NV 89557, USA (Received 16 October 1996; accepted 28 January 1997)
ABSTRACT The semi-continuousbiooxidationof a flotation concentrateof a particularly refractory Chinese gold ore was investigated. Withoutbiooxidation, even after roasting, it was possible to recover only a littlemore than 46% of the gold. The ore contained sulfide minerals, clay minerals, and carbonaceousmaterial.Approximately58% of the gold was associated with arsenopyrite, 38% with the clay minerals and about 7% with the carbonaceous material. Two different bacterial consortiawere studiedfor effectiveness at biooxidizingthe concentrate.Bothwere effectiveandarsenopyriteoxidationwasrapid. After the biotreatmentit waspossible to recover about 76% of the gold. Apparentlythe remaining non-recoverable gold is associated with the clays and the carbonaceous material. 0 1997 Published by Elsevier Science Ltd Keywords Gold ores, biooxidation, cyanidation
INTRODUCTION Ores whose gold recovery by direct cyanidation remains low after very fine grinding are called refractory gold ores. The cause of the refractoriness can be divided into two categories: mineralogical and chemical. Mineralogical re:fractoriness occurs when the Au particles are very fine, usually below a few pm in size, and locked within various minerals such as sulfide minerals. Because of the locking, the Au surfaces are not exposed to lixiviant solutions and are, thus, not dissolved. Chemical refractoriness occurs when there are detrimental substances present, such as arsenical or sulfide minerals which consume cyanide or oxygen needed for the dissolution process. Also, sometimes carbonaceous or clay minerals are present which interfere by removing Au from solution, often in the form of cyanide complexes. The Chongyang ore is both carbonaceous and disseminated with the Au locked in arsenopyrite. Thus, the ore is very difficult to treat. It is typical of several large Au ores in China [1,2,3] 577
Y. Wei et al.
578
ln the past, such an ore, both in China aud elsewhere has often been treated by roasting or pressure oxidation in front of conventional cyanidation. Roasting may cause substantial environmental problems and pressure oxidation requires the expenditure of large capital costs. Because of these problems with the noted treatments, experiments have been run on the biooxidation pretreatment of the Chongyang ore. REVIEW OF TANK BIOOXIDATION OF ARSENICAL REFRACTORY
GOLD ORES
The tank biooxidation process was developed in the former USSR [4]. Subsequently, the process has been successfully applied industrially in South Africa, Brazil and Australia [5,6]. Compared with other pretreatment methods the biooxidation process has the following advantages: lower capital costs, easy adjustment and control of operating parameters, often high gold recovery and relative freedom from environmental problems. The capacities of current operating refractory Au bio-reactors range from 40 to 720 tons of concentrate per day. The range of mineralogical compositions are as follows: arsenopyrite 10.9-20.7%, pyrite 6.5-36.9%, pyrrhotite up to 19 % . Thus, total sulfur varies between 11.4 and 20%) arsenic between 5 and 10 % and iron between 17.5 and 30%. The particle size of the concentrate is usually 75-90% minus 75 pm (200 mesh). Either one or two stage biooxidation is used. Volumes of single tanks vary from about 90 to 896 M3 and total volumes vary from 764 to 16,128 M3. Leaching time is usually less than 4 days and leaching pH is normally in the range pH 1.2- 1.8. Oxygen level in the pulp is maintained > 2mg/l. The bacteria strains are usually a consortium consisting primarily of Z?ziobacillusferrooxidans, i?ziobacillus thiooxidans and Leptospitillum ferrooxidans that have adapted to the biooxidation conditions present at a particular mine site. It is found that it normally takes between 2 and 2.5 years to develop a biooxidation process from a laboratory scale to the operating plant. Industrial tests show that when the oxidation of the sulfides reach 75-90%, cyanide recovery of Au is usually in the range 90-95 % (grade of concentrate 12-140 g/t). Reagent costs and electrical energy costs tend to be about l/3 each of the total costs of the operation. Most of the energy costs are for aeration and agitation of the pulp. CHONGYANG ORE AND RESEARCH METHODS USED The Chongyang carbonaceous and finely disseminated ore has the chemical composition shown in Table 1. Table 2 shows the mineral composition and Au distribution.
TABLE 1 Chemical analysis of chongyang ore
cu
I 0.007
MgO
1.52
I 4.32
II
Semi-continuous biooxidation of the Chongyang refractory gold ore
579
The sulfide minerals present are arsenopyrite, pyrite and stibnite. Most of the sulfide Au is in the arsenopyrite and very little is present in pyrite and stibnite. Clays constitute the major mineral fraction present in the ore and, although the concentration of Au is not great, overall the Au associated with the clay fraction amounts to about 38% of the total Au. Although the carbonaceous fraction is quantitatively small, about 7% of the Au is associated with this fraction. Note that the dolomite is harmful in that it consumes sulfuric acid in the process. Also note that the presence of antimony containing stibnite is harmful in that this element is a T. ferrooxidans toxin.
TABLE 2 Mineral composition and gold distribution
Mineral
Content (%)
Au Content in Single Mineral, g/t
Gold Distribution in Ore, %
Pyrite
2.92
<2
1.16
Arsenopyrite
0.59
400.69
53.59
Limonite
0.15
9.15
0.31
Stibnite
0.27
0.0096
0.007
Clay
54.2
3.08
37.84
Carbonaceous Material
0.28
110
6.98
Quartz
30.2
-
0.065
Dolomite
Direct cyanidation experimentation with the ore showed that with 4 kg/t NaCN, pH > 11 and a leaching time of 24 hr resulted in a Au recovery of only 15.8 % . Further, considering the low concentration of Au in the ore and the: carbonate concentration is high, the ore is not suitable for direct bioleaching and, thus, the sulfide and clay minerals must be preconcentrated by flotation. After grinding to 92 % minus 75 pm, a sulfide flotation concentrate containing 3 1.04 g/t Au, 9.48 % sulfur, 10.54% total Fe, and 2.32% arsenic was obtained. However, without bioleaching a maximum recovery of Au of only 40.2% was obtained by direct cyanidation. Even after roasting, with 91% of the arsenic removed, gold recovery was still only 46.2%. Set up for Semi-Continuous Biooxidation The biooxidation scheme was drawout-transfer-feed. The set up consisted of four one liter flotation tanks connected in series. The first tank was used for preparing the pulp and addition of an iron free 9K nutrient medium. Each tank was aerated using a micro air pump. The volummetric flow rate was 640 ml/min. Temperature was maintained between 28 and 3OOC.Agitation in the flotation tank was maintained at 760
580
Y. Wei et al.
rpm (propeller tip velocity about 2 m/set). Biooxidation Methods During the biooxidation process, pH, Eh, Fe, and arsenic in the liquid phase, total biomass and biomass activity were monitored daily. Arsenic in the solid phase was measured at irregular intervals. Fe2+ and total Fe in the pulp were determined by dichromate titration. Fe3+ was obtained by difference. Arsenic was determined by iodiometric titration. To measure the oxidized/precipitated Fe and arsenic concentrations the washed, dried and weighed precipitate was treated with 1: 1 HCl for 10 min and then rewashed with distilled water until the pH of the effluent water reached pH 5-6. Fe and arsenic concentration was then measured in the wash solution and the remaining solid Fe and arsenic were considered to be oxidized and precipitated. Biomass weight was measured by the cell wet method. In the procedure a sample of pulp was taken from the tank and centrifuged at 2,000 rpm for 20 min. The supematant was then taken and the pH adjusted to pH 0.8-l .Ousing a 1:4 HCl solution. Then, this solution was centrifuged at 6,500 to 8,000 rpm for 30 min. Finally, the clear solution left was removed and the centrifuge tube plus residue was oven dried for 30 min at 60°C. The weight of the cells was then determined by difference in weight of the empty tube and the tube containing biomass. Cell numbers in the pulp (N) was then calculated from the following relationship:
N = rnlx3.8~10~ (cells/ml)
The activity of the bacteria was determined by the speed of oxidation of Fe2+ (g/l.hr).
RESULTS Obtaining an Effective Culture Two bacterial cultures were prepared for the biooxidation of the concentrate. One culture (WICT) was isolated from a copper mine in China and had been adapted to the Chongyang concentrate by exposure to it over a period of about 2 yrs. The other was a culture from the former USSR. Flask tests were performed to select the best culture. To compare results for each culture three tests were performed. All tests were performed using 10 g of concentrate, 90 ml 9K medium and 10 ml of prepared bacterial culture (lo7 to 108 cells/ml). One test was performed without the addition of Fe2+. A second was performed in the presence of 9 g/l F$+ (through the addition of FeS0,.7H,O). The third test was a blank containing the, the concentrate, the medium, including Fe2+, a disinfectant (thymol), but no added bacterial culture. Without the presence of the bacteria the pH increased and there was no biooxidation of the sulfides. Table 3 shows results obtained for both cultures in the presence of added Fe2+ and depicts the biooxidation as expressed by the decrease in Fe2+ in the solution and by the amount of Fe3+ appearing in the solution. Table 4 shows similar results for the system without added Fe*+. It can be seen, regardless of the presence or absence of F$ +, that the Russian culture appeared superior in oxidizing ability. For example, from Table 3, it took 4 days for the WICT culture to obtain the comparable oxidation of F$+ achieved in 48 hr by the Russian culture. However, both cultures were active and, thus a mixture both cultures was used in subsequent semi-continuous biooxidation tests.
Semi-continuous biooxidation of the Chongyang refractory gold ore
581
TABLE 3 Results obtained with the cultures in the presence of Fez+ WICT Fe2+,
Fe3+,
Total
g/l
g/l
Fe
8.14
9.61
9.39
14.02
7.14
12.44
5.32
2.03
1 0.23
1 5.31
2.04
0.24
5.32
5.46
1.74
0.23
5.31
1.54
0.11
6.00
1 2.02
1 655
8.03
1 0.23
1 7.46
1.82
0.68
3.28
3.96
5.44
1.82
0.11
2.61
2.72
6.11
1.75
0.22
2.71
2.93
1 5.44
1 1.86
1 755
1 17.69
TABLE 4 Results obtained with the cultures in the absence of added FeZ’
Scheme of Semi-Continuous Bioleaching Tests The experimentation
was run in a series of tanks. Each tank had an effective volume of 0.8-0.85 1. The first tank was used for enrichment and concentrates were added to it at a solid/liquid ratio of 1:4. Initial pH was pH 2.5 and sulfuric acid was added to the pulp in order to lower pH to about pH 1.9. Initial iron content was Fe2+ = 0.9 g/l and Fe 3+ = 7.13 g/l and the initial bacteria activity was 0.12 g/l.hr. After several days most of the F$+ was oxidized and bacterial activity increased to 1S-3 .O g/l.hr. After 6 days the first tank was fully filled and the pH was pH 1.5, Eh was 699 mV and bacterial activity was 1.5 g/l.hr. After the first tank was filled the following tanks were slowly filled. This process was the first stage of oxidation, i.e., the drawout-fill-feed stage.
Y. Wei et al.
582
Parameters of Biooxidation In the first stage of biooxidation pH decreased from 2.28 in the first tank to 1.64 in the last one and Eh increased from 666 to 781 mV. Iron in the liquid phase was mostly in the form Fe3+ with concentrations varying from 2.3 to 4.2 g/l according to tank sequence, first to last. Biomass weight increased from 3.6 to 5.75 g cells/l. Bacteria activity ranged from 0.5 to 1.83 g/l.hr. The lowest pH in the series was pH 1.3 and the highest Eh 809 mV. The initial arsenic concentration in the liquid phase was 0.5 g/l and the concentration in the 4th (last tank) was 1.2 g/l. The amount of precipitated arsenic was between 2 and 6 times that present in the liquid phase. The distribution of iron between the solid and liquid phases paralleled that of arsenic (although in greater concentrations in both phases). Since the concentration of arsenic in the solid phase greatly decreased in the tanks it can be deduced that it is very easy to oxidize this element in the biooxidation process. After the system became stable, the biooxidation process reached the second stage, i.e., the stabilized and monitoring stage. At this point the feed rate was 80-120 g/l, introduced in 2 or 3 increments. Analytical analysis of this stage generated the results shown in Table 5. From the data of Table 5 it is seen that about 80% of the arsenic had been oxidized in the first tank and 95 % overall. Biooxidation time was 3-4 days. Subsequent Au recovery from the concentrate by cyanidation was 76%) considerably better than the 40% without treatment and the 46% with roasting. Other experimentation indicated that further grinding did not increase Au recovery. It is likely that some of the Au associated with the clay minerals and, especially, the carbonaceous material does not respond to biotreatment. TABLE 5 Results from the semi-continuous experiment (As in feed: 2.32%)
CONCLUSIONS For the Chongyang refractory Au ore concentrate containing 2.32 % arsenic and 3 1.04 g/t, Au semicontinuous biooxidation experiments show that 95 % of the arsenic can be oxidized within 3 to 4 days by such treatment. Although only about 40% of the Au could be recovered from the concentrate by cyanidation without pretreatment and only 46% recovered after roasting and cyanidation, it was possible to recover 76% of the Au after biooxidation and cyanidation. However, even with further grinding, additional Au recovery was not possible, probably because of the clays and carbonaceous material present in the ore. A biooxidation scheme with operating parameters has been developed for the treatment of Chongyang type ores.
Semi-continuous biooxidation of the Chongyang refractory gold ore
583
REFERENCES 1. 2. 3. 4. 5. 6.
Bin, I. 6c Jin, K., Progress in Treating Refractory Gold Ores, Hydrometullurgy, 8 (2), 33-40 (1988) (in Chinese). Tong, D.., The Latest Development and Future Prospect of Gold Extraction Technology, Metallic Ore Dmsing Abroad, 30(10), 29-35 (1993) (in Chinese). Anon, Report of Benejkiation and Extraction of Chongyang Gold Mine, Hubei Institute of Geology, Wuhan, Hubei, Peoples Republic of China, (1992) (in Chinese). Karavaiko, G.I. & Rossi, G., (editors), Biogeotechnology of Metals, Manual, Center for International Projects GKNT, Moscow, (1988) Brevis, 1’., Metal Extraction by Bacterial Oxidation, Mining Magazine, 173(4), 197-207 (1995). Dew, D.W., Miller, D.M. & van Aswegen, P.C., Genmin’s Commercialization of the Bacterial Oxidation Process for the Treatment of Refractory Gold Concentrate, presented at the Rand01 Gold Forum, IBeaver Creek ‘93, Vail, Colorado, USA, (Sept., 1993).
PII:SO892-6875(97)00037-X
Engineering, Vol. 10, No. 6, pp. 517-583, 1997 8 1997 Published by Else&r Science Ltd Printed in Great Britain. All rights resewed 0892-6875/97 SI7.00+0.00
SEMI-CONTINUOUS BIOOXIDATION OF THE CHONGYANG REFRACTORY GOLD ORE
Y. WEI§, K. ZHONGS, E.V. ADAMOVT and R.W. SMITH* 0
Wuhan Institute of Chemical Technology, 430074, P. R. China t Moscow Institute of Steel and Alloy, 117936, Moscow, Russia $ University of Nevada, Reno, NV 89557, USA (Received 16 October 1996; accepted 28 January 1997)
ABSTRACT The semi-continuousbiooxidationof a flotation concentrateof a particularly refractory Chinese gold ore was investigated. Withoutbiooxidation, even after roasting, it was possible to recover only a littlemore than 46% of the gold. The ore contained sulfide minerals, clay minerals, and carbonaceousmaterial.Approximately58% of the gold was associated with arsenopyrite, 38% with the clay minerals and about 7% with the carbonaceous material. Two different bacterial consortiawere studiedfor effectiveness at biooxidizingthe concentrate.Bothwere effectiveandarsenopyriteoxidationwasrapid. After the biotreatmentit waspossible to recover about 76% of the gold. Apparentlythe remaining non-recoverable gold is associated with the clays and the carbonaceous material. 0 1997 Published by Elsevier Science Ltd Keywords Gold ores, biooxidation, cyanidation
INTRODUCTION Ores whose gold recovery by direct cyanidation remains low after very fine grinding are called refractory gold ores. The cause of the refractoriness can be divided into two categories: mineralogical and chemical. Mineralogical re:fractoriness occurs when the Au particles are very fine, usually below a few pm in size, and locked within various minerals such as sulfide minerals. Because of the locking, the Au surfaces are not exposed to lixiviant solutions and are, thus, not dissolved. Chemical refractoriness occurs when there are detrimental substances present, such as arsenical or sulfide minerals which consume cyanide or oxygen needed for the dissolution process. Also, sometimes carbonaceous or clay minerals are present which interfere by removing Au from solution, often in the form of cyanide complexes. The Chongyang ore is both carbonaceous and disseminated with the Au locked in arsenopyrite. Thus, the ore is very difficult to treat. It is typical of several large Au ores in China [1,2,3] 577
Y. Wei et al.
578
ln the past, such an ore, both in China aud elsewhere has often been treated by roasting or pressure oxidation in front of conventional cyanidation. Roasting may cause substantial environmental problems and pressure oxidation requires the expenditure of large capital costs. Because of these problems with the noted treatments, experiments have been run on the biooxidation pretreatment of the Chongyang ore. REVIEW OF TANK BIOOXIDATION OF ARSENICAL REFRACTORY
GOLD ORES
The tank biooxidation process was developed in the former USSR [4]. Subsequently, the process has been successfully applied industrially in South Africa, Brazil and Australia [5,6]. Compared with other pretreatment methods the biooxidation process has the following advantages: lower capital costs, easy adjustment and control of operating parameters, often high gold recovery and relative freedom from environmental problems. The capacities of current operating refractory Au bio-reactors range from 40 to 720 tons of concentrate per day. The range of mineralogical compositions are as follows: arsenopyrite 10.9-20.7%, pyrite 6.5-36.9%, pyrrhotite up to 19 % . Thus, total sulfur varies between 11.4 and 20%) arsenic between 5 and 10 % and iron between 17.5 and 30%. The particle size of the concentrate is usually 75-90% minus 75 pm (200 mesh). Either one or two stage biooxidation is used. Volumes of single tanks vary from about 90 to 896 M3 and total volumes vary from 764 to 16,128 M3. Leaching time is usually less than 4 days and leaching pH is normally in the range pH 1.2- 1.8. Oxygen level in the pulp is maintained > 2mg/l. The bacteria strains are usually a consortium consisting primarily of Z?ziobacillusferrooxidans, i?ziobacillus thiooxidans and Leptospitillum ferrooxidans that have adapted to the biooxidation conditions present at a particular mine site. It is found that it normally takes between 2 and 2.5 years to develop a biooxidation process from a laboratory scale to the operating plant. Industrial tests show that when the oxidation of the sulfides reach 75-90%, cyanide recovery of Au is usually in the range 90-95 % (grade of concentrate 12-140 g/t). Reagent costs and electrical energy costs tend to be about l/3 each of the total costs of the operation. Most of the energy costs are for aeration and agitation of the pulp. CHONGYANG ORE AND RESEARCH METHODS USED The Chongyang carbonaceous and finely disseminated ore has the chemical composition shown in Table 1. Table 2 shows the mineral composition and Au distribution.
TABLE 1 Chemical analysis of chongyang ore
cu
I 0.007
MgO
1.52
I 4.32
II
Semi-continuous biooxidation of the Chongyang refractory gold ore
579
The sulfide minerals present are arsenopyrite, pyrite and stibnite. Most of the sulfide Au is in the arsenopyrite and very little is present in pyrite and stibnite. Clays constitute the major mineral fraction present in the ore and, although the concentration of Au is not great, overall the Au associated with the clay fraction amounts to about 38% of the total Au. Although the carbonaceous fraction is quantitatively small, about 7% of the Au is associated with this fraction. Note that the dolomite is harmful in that it consumes sulfuric acid in the process. Also note that the presence of antimony containing stibnite is harmful in that this element is a T. ferrooxidans toxin.
TABLE 2 Mineral composition and gold distribution
Mineral
Content (%)
Au Content in Single Mineral, g/t
Gold Distribution in Ore, %
Pyrite
2.92
<2
1.16
Arsenopyrite
0.59
400.69
53.59
Limonite
0.15
9.15
0.31
Stibnite
0.27
0.0096
0.007
Clay
54.2
3.08
37.84
Carbonaceous Material
0.28
110
6.98
Quartz
30.2
-
0.065
Dolomite
Direct cyanidation experimentation with the ore showed that with 4 kg/t NaCN, pH > 11 and a leaching time of 24 hr resulted in a Au recovery of only 15.8 % . Further, considering the low concentration of Au in the ore and the: carbonate concentration is high, the ore is not suitable for direct bioleaching and, thus, the sulfide and clay minerals must be preconcentrated by flotation. After grinding to 92 % minus 75 pm, a sulfide flotation concentrate containing 3 1.04 g/t Au, 9.48 % sulfur, 10.54% total Fe, and 2.32% arsenic was obtained. However, without bioleaching a maximum recovery of Au of only 40.2% was obtained by direct cyanidation. Even after roasting, with 91% of the arsenic removed, gold recovery was still only 46.2%. Set up for Semi-Continuous Biooxidation The biooxidation scheme was drawout-transfer-feed. The set up consisted of four one liter flotation tanks connected in series. The first tank was used for preparing the pulp and addition of an iron free 9K nutrient medium. Each tank was aerated using a micro air pump. The volummetric flow rate was 640 ml/min. Temperature was maintained between 28 and 3OOC.Agitation in the flotation tank was maintained at 760
580
Y. Wei et al.
rpm (propeller tip velocity about 2 m/set). Biooxidation Methods During the biooxidation process, pH, Eh, Fe, and arsenic in the liquid phase, total biomass and biomass activity were monitored daily. Arsenic in the solid phase was measured at irregular intervals. Fe2+ and total Fe in the pulp were determined by dichromate titration. Fe3+ was obtained by difference. Arsenic was determined by iodiometric titration. To measure the oxidized/precipitated Fe and arsenic concentrations the washed, dried and weighed precipitate was treated with 1: 1 HCl for 10 min and then rewashed with distilled water until the pH of the effluent water reached pH 5-6. Fe and arsenic concentration was then measured in the wash solution and the remaining solid Fe and arsenic were considered to be oxidized and precipitated. Biomass weight was measured by the cell wet method. In the procedure a sample of pulp was taken from the tank and centrifuged at 2,000 rpm for 20 min. The supematant was then taken and the pH adjusted to pH 0.8-l .Ousing a 1:4 HCl solution. Then, this solution was centrifuged at 6,500 to 8,000 rpm for 30 min. Finally, the clear solution left was removed and the centrifuge tube plus residue was oven dried for 30 min at 60°C. The weight of the cells was then determined by difference in weight of the empty tube and the tube containing biomass. Cell numbers in the pulp (N) was then calculated from the following relationship:
N = rnlx3.8~10~ (cells/ml)
The activity of the bacteria was determined by the speed of oxidation of Fe2+ (g/l.hr).
RESULTS Obtaining an Effective Culture Two bacterial cultures were prepared for the biooxidation of the concentrate. One culture (WICT) was isolated from a copper mine in China and had been adapted to the Chongyang concentrate by exposure to it over a period of about 2 yrs. The other was a culture from the former USSR. Flask tests were performed to select the best culture. To compare results for each culture three tests were performed. All tests were performed using 10 g of concentrate, 90 ml 9K medium and 10 ml of prepared bacterial culture (lo7 to 108 cells/ml). One test was performed without the addition of Fe2+. A second was performed in the presence of 9 g/l F$+ (through the addition of FeS0,.7H,O). The third test was a blank containing the, the concentrate, the medium, including Fe2+, a disinfectant (thymol), but no added bacterial culture. Without the presence of the bacteria the pH increased and there was no biooxidation of the sulfides. Table 3 shows results obtained for both cultures in the presence of added Fe2+ and depicts the biooxidation as expressed by the decrease in Fe2+ in the solution and by the amount of Fe3+ appearing in the solution. Table 4 shows similar results for the system without added Fe*+. It can be seen, regardless of the presence or absence of F$ +, that the Russian culture appeared superior in oxidizing ability. For example, from Table 3, it took 4 days for the WICT culture to obtain the comparable oxidation of F$+ achieved in 48 hr by the Russian culture. However, both cultures were active and, thus a mixture both cultures was used in subsequent semi-continuous biooxidation tests.
Semi-continuous biooxidation of the Chongyang refractory gold ore
581
TABLE 3 Results obtained with the cultures in the presence of Fez+ WICT Fe2+,
Fe3+,
Total
g/l
g/l
Fe
8.14
9.61
9.39
14.02
7.14
12.44
5.32
2.03
1 0.23
1 5.31
2.04
0.24
5.32
5.46
1.74
0.23
5.31
1.54
0.11
6.00
1 2.02
1 655
8.03
1 0.23
1 7.46
1.82
0.68
3.28
3.96
5.44
1.82
0.11
2.61
2.72
6.11
1.75
0.22
2.71
2.93
1 5.44
1 1.86
1 755
1 17.69
TABLE 4 Results obtained with the cultures in the absence of added FeZ’
Scheme of Semi-Continuous Bioleaching Tests The experimentation
was run in a series of tanks. Each tank had an effective volume of 0.8-0.85 1. The first tank was used for enrichment and concentrates were added to it at a solid/liquid ratio of 1:4. Initial pH was pH 2.5 and sulfuric acid was added to the pulp in order to lower pH to about pH 1.9. Initial iron content was Fe2+ = 0.9 g/l and Fe 3+ = 7.13 g/l and the initial bacteria activity was 0.12 g/l.hr. After several days most of the F$+ was oxidized and bacterial activity increased to 1S-3 .O g/l.hr. After 6 days the first tank was fully filled and the pH was pH 1.5, Eh was 699 mV and bacterial activity was 1.5 g/l.hr. After the first tank was filled the following tanks were slowly filled. This process was the first stage of oxidation, i.e., the drawout-fill-feed stage.
Y. Wei et al.
582
Parameters of Biooxidation In the first stage of biooxidation pH decreased from 2.28 in the first tank to 1.64 in the last one and Eh increased from 666 to 781 mV. Iron in the liquid phase was mostly in the form Fe3+ with concentrations varying from 2.3 to 4.2 g/l according to tank sequence, first to last. Biomass weight increased from 3.6 to 5.75 g cells/l. Bacteria activity ranged from 0.5 to 1.83 g/l.hr. The lowest pH in the series was pH 1.3 and the highest Eh 809 mV. The initial arsenic concentration in the liquid phase was 0.5 g/l and the concentration in the 4th (last tank) was 1.2 g/l. The amount of precipitated arsenic was between 2 and 6 times that present in the liquid phase. The distribution of iron between the solid and liquid phases paralleled that of arsenic (although in greater concentrations in both phases). Since the concentration of arsenic in the solid phase greatly decreased in the tanks it can be deduced that it is very easy to oxidize this element in the biooxidation process. After the system became stable, the biooxidation process reached the second stage, i.e., the stabilized and monitoring stage. At this point the feed rate was 80-120 g/l, introduced in 2 or 3 increments. Analytical analysis of this stage generated the results shown in Table 5. From the data of Table 5 it is seen that about 80% of the arsenic had been oxidized in the first tank and 95 % overall. Biooxidation time was 3-4 days. Subsequent Au recovery from the concentrate by cyanidation was 76%) considerably better than the 40% without treatment and the 46% with roasting. Other experimentation indicated that further grinding did not increase Au recovery. It is likely that some of the Au associated with the clay minerals and, especially, the carbonaceous material does not respond to biotreatment. TABLE 5 Results from the semi-continuous experiment (As in feed: 2.32%)
CONCLUSIONS For the Chongyang refractory Au ore concentrate containing 2.32 % arsenic and 3 1.04 g/t, Au semicontinuous biooxidation experiments show that 95 % of the arsenic can be oxidized within 3 to 4 days by such treatment. Although only about 40% of the Au could be recovered from the concentrate by cyanidation without pretreatment and only 46% recovered after roasting and cyanidation, it was possible to recover 76% of the Au after biooxidation and cyanidation. However, even with further grinding, additional Au recovery was not possible, probably because of the clays and carbonaceous material present in the ore. A biooxidation scheme with operating parameters has been developed for the treatment of Chongyang type ores.
Semi-continuous biooxidation of the Chongyang refractory gold ore
583
REFERENCES 1. 2. 3. 4. 5. 6.
Bin, I. 6c Jin, K., Progress in Treating Refractory Gold Ores, Hydrometullurgy, 8 (2), 33-40 (1988) (in Chinese). Tong, D.., The Latest Development and Future Prospect of Gold Extraction Technology, Metallic Ore Dmsing Abroad, 30(10), 29-35 (1993) (in Chinese). Anon, Report of Benejkiation and Extraction of Chongyang Gold Mine, Hubei Institute of Geology, Wuhan, Hubei, Peoples Republic of China, (1992) (in Chinese). Karavaiko, G.I. & Rossi, G., (editors), Biogeotechnology of Metals, Manual, Center for International Projects GKNT, Moscow, (1988) Brevis, 1’., Metal Extraction by Bacterial Oxidation, Mining Magazine, 173(4), 197-207 (1995). Dew, D.W., Miller, D.M. & van Aswegen, P.C., Genmin’s Commercialization of the Bacterial Oxidation Process for the Treatment of Refractory Gold Concentrate, presented at the Rand01 Gold Forum, IBeaver Creek ‘93, Vail, Colorado, USA, (Sept., 1993).