AU654322B2 - Biological leaching of transition ores - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPEC I F I CA TION FOR A STANDARD PATENT

ORIGINAL

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ooanu tro~~~~ o Name of Applicant: Actual Inventors: o n 0 a o a 0 0^4^Q MOUNT ISA MINES LIMITED, A.C.N. 009 661 447 David Arthur Winborne and Fook-Sin Wong SHELSTON WATERS 55 Clarence Street SYDNEY NSW 2000 "LEACHING OF TRANSITION ORE" Address for Service: 0* 00 Invention Title: Details of Associated Provisional Application No: PK4848 The following statement is a full description of this invention, including the best method of performing it known to us:- -2- The present invention relates to a method of recovery of one or more valuable metals from zinc transition ores and more particularly to leaching of zinc transition ores in-situ, in heap, dump or in-ground. A transition ore is the oxidized upper layer of an ore body.

It is generally accepted that methods applicable to unoxidized zinc ores are not applicable to zinc transition ores. For example, while sphalerite may be concentrated by flotation prior to metal extraction, the corresponding transition ore is not amenable to recovery by flotation. Attempts to sulphidise the oxidized surface of a transition ore to enable concentration by flotation have been unsuccessful. Other proposed chemical treatments of the transition ore to recover zinc have involved excessive reagent consump~tion. A particular problem of treatment of transition ores in-situ or in heap is the tendency for iron present in the ore or in treating liquor to precipitate as an 0.20 insoluble precipitate leading to percolation problems.

Consequently there is a need for a process which is able to recover zinc from zinc transition ore by economically viable means and it is an object of the present invention to provide such a method.

According to the invention a method of extracting one or more valuable metals from a zinc transition ore by biological leaching of the ore wherein the leaching may be heap, dump, or in-situ. leaching.

71 3- In a preferred embodiment, the transition ore is zinc transition ore and biological leaching includes the use of microorganisms which may include T. Ferrooxidans or L. Ferrooxidans, T. Thiooxidans, heterotrophic organisms or mixtures thereof.

In a particularly preferred embodiment, the ore is sphalerite and a mixture of T. and L. Ferrooxidans and T.

Thiooxidans is used.

Preferably, the pH is in the range of 1.6 to most preferably 2.0, and the temperature is in the range of 25 to 350C, most preferably 30 0

C.

In a further aspect of the present invention, there is provided in accordance with the invention a method of extracting an iron product from a leach solution comprising the step of reacting the leach solution with a reactant to precipitate an iron product. Preferably, the reactant is lime or limestone and the iron product is precipitated as ferric iron Fe(OH) Preferably the 3 S' pH is between 2.0 and 5.0, most preferably Preferably, zin- is extracted from the reaction product by solvent extraction after iron removal. More preferably, the solvent is di-ethyl hexyl phosphoric acid (D2EHPA).

Preferably, pH during solvent extraction is maijtained at 3.0-5.0. This may be done using ammonia or sodium hydroxide. A preferred embodiment of the invention will now be described with reference to the Examples wherein: 4 ^t 5

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a e o o oa os D o o~o o 0 as e o~e a 0 0 6 oo e~so oo~ aoo u 0 6 a Figure 1 is a oraph of a column leach trial graphed as Zn extraction versus time.

In practice, a range of ores may be presented for processing. Such ores include transition ores or sphalerite, i.e. the ores contain a high degree of Fe, preferably in the form of pyrite. The ore may be built into a heap and treated as for a heap leach situation.

Alternatively, the ore may be treated in-ground by in-situ leaching.

10 As a preliminary step to the recovery of zinc from a bioleach liquor by the method of bioleaching transition ore of the present invention, ferric iron is precipitated from an acidic sulphate leachate using limestone or lime. This preliminary step also precedes 15 solvent extraction of zinc by the use of di-ethyl hexyl phosphoric acid.

Transition ore contains the oxidation products of sphalerite (ZnS) and these generally include ZnSO ZnCO ZnO and ZnO A considerable amount of sphalerite remains unoxidisd.

Chemical leaching of zinc from transition ore is believed to occur by the following steps: Chemical attack on oxidised products: ZnCO H SO ZnSO H 0 CO 3 2 4 4 3 2 ZnO H SO ZnSO H O.

2 4 4 2 Together with the zinc sulphate already present, chemical dissolution is believed to account for up to 20% of zinc dissolved.

5 The dissolution of sphalerite is believed to occur by ferric sulphate dissolution. Ferric sulphate is the product of bacteria Thiobacillus Ferrooxidan, or Leptospirillium Ferrooxidan assisted pyrite oxidation, as follows:

T.F./L.F.

FeS 2 7/202 H 2 FeS0 4

H

2 S0 4

T.F.

2FeS04+ H 2 S0 4 +1/2 0 2 Fe 2 (S0 4 3

H

2 0.

Pyrite oxidation may also occur via the indirect route of ferric sulphate action: FeS 2 Fe 2 (S0 4 3 3 FeSO 4 2S.

The sulphur is further oxidised to sulphuric acid by the bacteria Leptospirillium Ferrooxidan and Thiobacillus Thiooxidan: 2S 302 2H 2 0----2H 2

SO

4 i ZnS is oxidised by ferric sulphate according to 20 the equation: ZnS Fe 2 ZnSO 4 2FeSO 4

S.

2 4 3 4 4 Combined with sulphur oxidation, the overall reaction is: i 2 ZnS 2 Fe 2

(SO

4 3 3 02 2H20---2 ZnSO 4 4 FeSO 4 2 H2S04* L i i. LC _iii jt~Y i f-i'.

r:- 6 Testwork has examined bacterial leaching of zinc from transition ore under optimum conditions of nutrient, aeration, temperature, and particle size. The results showed over 95% zinc extraction from the transition ore.

Testwork has also examined column leaching of zinc from a partially oxidised transition ore. This was also conducted under optimum conditions of aeration, temperature and particle size. Mineralogical studies show the bulk of the zinc in the transition ore to exist as sphalerite with a minor proportion of ZnO 2 There are several solvents for the extraction of zinc from an acid sulphate liquor, but only D2EHPA has been used commercially for zinc recovery. The acid sulphate liquor containing zinc and ferric iron is treated with limestone to raise the pH to 3.0 to precipitate the ferric iron as ferric hydroxide. The resulting calcium sulphate and ferric hydroxide are removed by filtration. Zinc is extracted at pH 3.0 using D2EHPA. Ammonia is added to maintain the preferred pH.

20 Zinc is stripped with sulphuric acid.

EXAMPLE 1 Ore: Hilton transition ore Grade: Two grades: High and low Ore preparation: 25 Ring-ground to 0.4mm diameter (pulverised) PEH_ Temperature: r;s Maintained at 300C 99 oa-o o 4 o040 9 o00 0o 0 oaa oo o 0 00 a o o aaa 00 *a 0 0 0 "0 0: o o oo 000 oi a O o 0 0 0 0 00* a 0 oo o o a oa ua a 9 or c Hrf] A/ V 1A

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T 0 ~I 7 Bacteria: T. Ferrooxidan Microbiological preparation: Three types of water were cultured 1) Mine water from Mt Isa Mines (MIM) 2) Pit water from Collinsville Coal Mines (CCP) 3) 50% mixture of 1) and 2).

2 x 800ml of 2) and 3) were added to a beaker together with 9K nutrients (3g/L (NH 4 2

SO

4 K2HPO4, 0.5g/L MgSO4, 0.1g/L KC1, 0.01g/L

NH

4

NO)

3 together with 2g/L Fe 2 as FeSO 4 .7H20 02- a aS0 and Ig/L SO 2 as Na SO 2 3 2 2 3 The beaker was aerated and the bioactivity of the resultant solution monitored.

Water activity was monitored by the rate of Fe 3 formation at pH 2.0 of cultured liquor solution.

When the solution had reached a high level of activity, usually a period of four days, the resultant 4; solutions were used for the following leaching experiments.

The ore material was introduced stepwise at 5, 20% slurry .ensity over a period of 3 months to allow gradual bacteria adaptation to increasing solids density. pH was monitored and where pH had risen too high above 2.0 (due to consumption by dolomite), sulphuric acid was added to return the pH to near Samples were taken at intervals to monitor extractio.

1 /T 'A ;p -h_ F 8 reaction mixture was removed, and membrane-filtered. Solids returned to the reactor and filtrate assayed. Volumes were raintained to the starting volume using deionised water.

The results are detailed in Tables 1 and 2.

All three types of waters appeared to perform equally well on the high grade material extracting over 96% of the zinc content. On the low grade material, however, the CCP water achieved a higher zinc extraction at 87% over Isa water at 52 65%. The lower extraction with Isa water appeared to be an anomaly as a mixed culture extracted 95% of this zinc content in the low grade ore. Silver was not extracted. 4 ~r~U-v i TABLE 1: BIOLEACHI OF HILTON TRANSITION ORE L/CCP L/ISA L/MIX II/CCP If/ISA Il/MIX Slurry -Time pH Zn pi1 Zn pIl Zn pl Zn pli Zn pli Zn Density* (Days) Leached Leached Leached Leached Leached Leached 0 2.0 2.0 2.0 2.0 2.0 1 32 35 28 19 20 4 1.82 56 1.78 53 1.87 49 1.60 35 1.77 33 1.79 8 72 67 65 53 44 43 11 39 67 70 72 69 68 57 40 2.72 3.33 2.60 1.37 1.5 1.67 40 2.09 2.10 2.07 47 2.50 66 3.40 59 2.66 66 1.43 57 1.94 62 2.09 56 2.41 3.24 2.51 1.36 1.92 2.04 2.16 2.21 2.16 61 77 66 82 69 69 69 63 2.75 2.68 2.44 1.33 1.83 2.03 63 2.02 2.00 1.90 63 2.30 76 3.30 58 2.90 75 1.58 61 2.57 63 ,2.35 57 70 2.02 2.00 2.02 1.82 1.98 82 2.07 87 2.58 67 2.'9 83 1.36 73 1.70 73 2.02 69 89 2.08 92 2.60 66 2.23 79 1.23 1.56 78 1.94 73 96 1.93 88 2.44 65 2.03 79 1.24 il 1.28 77 1.66 74 97F 86.7 52.2 95.2 98.2 97.5 95.1 Zn Fe Pb Cu Mg Ca Ag ppm Ore Grade: L Low Grade II high Grade F Based on Final Residue 5.54 14.6 17.1 22.1 2.28 2.43 0.0133 2.32 0.0138 0.143 50% ISA/CCP 1.25 1.71 37 112SOq added to adjust pi. 4 x 50g in 800mnL ISA Isa water, CCP Collinsville water. Mix 10 TABLE 2: BIOLEACH RESIDUE GRADES/LIQUOR ASSAY 44 44 4 4 44 444, 4 4 4,, b4 I Zn Fe ?b Cu Mg Ca Ag Wt ppm (g) Ore: L 5.54 17.1 2.28 0.0133 1.26 2.32 37 H 14.6 22.1 2.43 0.0138 0.171 0,143 Residue: L/CCP 0.38 13.8 2.39 0.0115 0.182 2.67 38 195.3 L/ISA 1.22 15.0 1.89 0.618 0.227 2.09 36 217.1 L/MIX 0.140 14.3 2.02 0.345 0.203 2.31 38 189.5 H/CCP 0.447 14.5 3.97 0.0123 0-0340 0.0415 90 115.6 H/ISA 0.508 15.7 3.16 0.262 0.0354 0.0057 77 13,43.9 H/MIX 0.901 17.0 2.87 0.210 0.0399 0.0080 67 159.3 Leach Liquor: g/L Acid consumption (kg/t) CCP 0.04 3.8 0.0006 0.004 0.58 0.68 ISA 0.4 8.1 0.011 5.0 1.3 0.36 L/CCP 12.2 4.8 0.001 0.015 2.5 0.4 L/ISA 9.0 0.49 2.0 2.0 2.6 0.3 5.2 L/MIX 10.9 3.3 0.003 1.1 2.4 0.4 5.3 H/CCP 26.0 27.6 0.008 0.028 3.1 0.5 0 H/ISA 28.1 20.0 0.003 3.2 1.3 0.4 H/MIX 26.9 11.7 0.002 1.7 1.1 0.4 1.2 q Theoretical based on pH changes

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e *0 ao o ea 0 0 a a a a4 4 a 11 EXAMPLE 2. Column Leach Trial to Simulate Heap Leaching.

Transition ore material tested was from a percussion bore hole (J498) at Hilton. Samples from a zinc-containing band (63-94 metres) was composited and the very fine material (-100 mesh) was removed by screening, from an already fine bulk caused by percussion drilling.

The material (3.79% Zn) was packed into a column OD x 1.7M) and pretreated with sulphuric acid to 10 consume the acid consumers. 50 g/l sulphuric acid was trickled down the column until the pH had dropped below Sulhuric acid of pH 1.8 was next used until exiting liquor at the bottom of the column was also of pH 1.8. 8.5% zinc was extracted by the acid washing.

The column was then inoculated with a mixed bacterial culture the culture containing bacteria naturally occuring in water. 15L of bacterial inoculum was circulated through the column at lOmL/min for 2 days. The mixed bacterial culture was prepared by inoculating 170L of ferrous sulphate solution (2 g/L Fe 2 at pH 1.8 with 20 L of mine water from Mount Isa Mines. The cultured liquor (pH readjusted to 1.8) was used when a high level of bacterial activity had been reached, indicated by a total conversion of ferrous ions to ferric ions. 0.9% of the zinc content was extracted during column inoculation. The column was then continually washed with pH 1.8 sulphuric acid solution on a once-through basis for 290 days. Trickle flow rate was fia bl rl; bx; 12 maintained at 2 mL/min and a continuous suction applied at the bottom of the column to assist with aerating the column. The exiting liquor continually was assayed and volumes recorded. The results are summarised in Table 3 and Figure 1.

A total of 81% Zn was extracted over 290 days.

Approximately 8.5% of the Zn appeared oxidised and was solubilised in the initial acid o i r r ~rrt e ro o eroarr oau~n* s~ r P 11

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o o 0 a 4 o o 0o O ft 0 a 0 000 0 e o 0 -f .303t 03 3< 00 00 0 0 0 0 00 o o 00* 0 00 00 00 fI P0 0 TABLE 3: COLUM LEACHIHG TRIAL RESLTS LIQUOR ASSAYS TIME pH BACTERIAL Fe++ Fe total Zn Ca Mg Sulphate (days) COUNTS (mgfL) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (x 10 000 000) (correcced) water water Pretreat 19 13 Inocula PERCENTAGE EXTRACTION SOLIDS Fe Zn Pb Ca Mg Zinc Iron Solids Fe Zn Recovery Recovery Weight (g/day) (glday) (kg) 4.05 8.53 0 3.20 1.07 61.11 140.36 .57 .88 0 .26 .09 6.30 19.95 1 2.44 285 3 2.08 0 4 2.25 .46 2.28 .17 6 2 .08 7 2.08 .19 8 2.14 .56 9 2.31 .76 12 2.51 .08 13 2.55 .12 14 3.50 .18 18 2.22 .51 24 2.73 .13 27 1.91 0 31 1.79 .07 33 1.80 0 37 1.71 1.67 .08 /4 1.50 0 47 3.10 .54 51 1.94 .55 54 3.04 1.03 58 3 .33 61 3.04 .31 1090 220 330 200 170 200 190 250 220 170 135 60 100 280 220 280 240 330 310 280 20 210 270 224 30 148 70 65 60 59 20 68 10 27 90 82 30 37 70 67 60 210 30 54 80 64 230 190 18.90 18.20 3.79 235 183 21 175 20 223 33 170 18 97 105 135 32 276 21 7 280 21 700 19 860 19 150 16 60 17 130 33 21 130 15 17 28 1500 33 160 19 2100 33 2300 23 1700 23 4.26 9.29 0 9.41 2.17 1.09 1.26 8.29 18.56 0 172.76 14.79 3.50 7.28 8.44 21.61 0 169.18 15.56 3.12 .74 8.47 21.79 0 165.01 1,22 .14 .13 8.51 22.33 0 162.07 16.87 .55 8.55 22.51 0 160.95 17.70 .18 .19 8.63 24.37 0 159.77 18.63 3.34 .67 8.72 29.14 0 157.78 19.27 11.39 1.01 -340 2580 3120 2170 4 TIME pH BACTERIAL 0> 0 0 0e 00 0o 0o 0 0 0o 0 "AHLE 3: COLMI LEAC1HG TRIAL RESULTS (CONTIIUED LIQUOR ASSAYS PERCENTAGE EXTRACTION Fe++ Fe total Zn Ca mg Sulphate Fe Zn Pb Ca Rg Zinc (mg/L) (mg/L) (mg/L) (mg/L} (mgiL) (mg/L) M% Recove (corrected) (g/day 714 590 1300 21 12 1890 209 "190 3900 19 13 3000 272 220 1800 19 13 1680 8.80 33.99 0 156.21 19.87 2.90 56 43 1020 17 12 -3340 8.98 37.86 0 154.32 20.48 13.86 71 540 1900 13 13 4280 279 260 1200 18 13 710 9.30 42.73 0 151.65 20.94 4.98 251 330 1400 19 14 2140 302 380 1300 19 12 2030 9.55 47.21 0 149.93 21.52 4.59 352 440 1300 18 12 1900 9.77 50.79 0 148.55 21.97 6.41 (days) 68 73 79 82 86 89 93 97 100 103 110 114 117 121 124 128 131 135 138 143 145 149 152 156 159 163 166 170

COUNTS

(x 10 000 000} 2.90 3.01 2.93 2.41 1.38 2.02 2.13 2.63 2.34 1.75 1.60 2 1.70 2.26 2.21 1.90 1.57 1.58 2.07 1.85 1.84 -1.65 1.62 1.62 1.56 1.58 1.83 1.72 1.47 1.80 .06 .12 .14 .33 .57 1.53 .75 .68 .89 1.43 1.42 .79 2.53 4.28 4.12 2.88 6.82 2.69 .55 1.19 20.23 45.74 15.87 1.60 53.06 33.75 1.72 2.77 45.93 25.61 SOLIDS Iron Solids Fe Zn ry Recovery Height (g/day) (kg) .24 3.07 1.57 1.22 1.92 .49 .42 1.50 2.32 0 0 80 0 80 101 221 20 0 302 181 171 0 0 0 0 0 90 151 0 281 73 57 270 130 360 450.

540 150 51 540 420 340 42 29 31 23 26 340 340 34 53 1200 1300 1200 63 1000 .1300 1300 1200 '27 1400 1400 1100 14 7 13 6 5 880 900 19 1000 770 280 1780 5o 3170 1640 1090 320 4820 4630 1720 -160 80 190 290 180 2010 2660 -190 3170 9.87 51.24 147.29 22.63 .46 10.00 10.17 10.37 52.65 54.67 57.22 145.85 23.16 1.01 143.22 23.66 3.61 141.89 24.20 6.10 141.00 24.97 2.14 10.61 59.31 10.72 62.30 10.91 65.72 10.99 66.11 11.01 66.13 11.05 66.60 11.22 68.22 1.15 139.53 25.61 3.06 138.94 26.15 3.50 137.70 26.53 137.32 27.01 .02 136.82 27.53 .48 136.11 27.94 1.66

F

~~ri *0 *0 o 0 0 0 0 04 0, 0 0 0 04 0 0 0 O a a a a 000 *00 400 0 0*3 TABLE 3: COLUMN LEACHING TRIAL RESULTS (CONTINUED LIQUOR ASSAYS PERCENTAGE EXTRACTION -SOLIDS ASSAYS TIME pH BACTERIAL Fe++ (days) COUNTS (mglL) (z 10 000 000) Fe total Zn Ca (mg/L) (mg/L) (mg/L) Mg Sulphate (mg/L) (mg/L) (corrected) Pb Ca Mg M M M Zinc Recovery (g/day) Iron Solids Recovery Weight (g/day) (kg) 1.64 1.57 1.68 1.69 1.67 1.65 1.72 1.61 1.59 45.95 202.22 315 237.54 344.80 33.05 22.45 16.90 50 -220 110 140 150 180 160 -1320 0 11.42 69.70 11.43 11.44 11.44 69.73 69.74 69.75 1.71 13.39 1.86 47.32 1.79 1.35 1.94 1.13 1.98 3.06 0 12 78 300 11.45 69.77 11.45 69.77 11.46 69.77 11.46 69.81 11.47 70.27 11.50 71.23 135.70 28.41 1.51 134.59 28.71 .02 134.01 29.07 .03 133.18 29.39 .02 130.73 29.24 .02 128.71 29.11 0 127.01 29.00 .00 125.32 28.88 .10 123.64 28.77 .30 122.41 28.68 2.31 120.81 28.57 .24 119.61 28.48 .02 117.91 28.36 .89 116.48 28.25 2.06 115.22 28.16 1.43 114.10 28.07 1.21 112.96 27.98 1.71 111.84 27.90 1.48 11.57 11.57 11.68 12.06 12.47 12.63 13.30 13.58 71.47 71.48 72.35 75.22 76.62 77.80 79.47 80.92 .03 .09 .03 .33 .52 2.04 .78 3.27 1.40 1.85 1.34 22 43 18 oo00oo---

V

16 EXAMPLE 3 A synthetic leach liquor at pH 2 containing 11.1 g/L Zn, 3.2 g/L Fe, 0.01 g/L Cu, 2.5 g/L Mg and 0.3 g/L Ca was treated with limestone to pH 3.0 to remove ferric iron before being subjected to solvent extraction using 12% di-2-ethylhexyl phosphoric acid (D2EHPA) in Shellsol 2046 a Kerosene manufactured by Shell Co. as diluent in bench shakeout tests. Ferric level was reduced from 3.2 o l g/l to 0.01 g/L. The liquor was then solvent extracted o o S10 twice using an O/A (organic:aqueous) ratio of 2:1.

o *e The results are summarised in Table 4 below.

TABLE 4: ZINC RECOVERY BY SOLVENT EXTRACTION ou Zn D(Zn) Fe Cu Mg Ca(g/L Leach Liquor 11.1 3.20 0.01 2.50 0.30 Leach Liquor (pH 3.0) 11.5 0.01 0.01 2.40 0.30 Raffinate 1 0.80 6.69 0.00 0.01 2.90 0.20 Raffinate 2 0.00 0.00 0.01 1.90 0.00 D(Zn) Dissociation constant Stripping to recover the loaded zinc in the organic phase was achieved using 200 g/L sulphuric acid solution. O/A ratio used was 3.5:1 and two strip stages were sufficient for complete zinc recovery. 86% of the loaded zinc was stripped in the first strip stage. The strip liquor contained 16.2 g/L Zn.

-t #1i 17 Under optimum bioleaching conditions of grind size, nutrients, temperature and pH, over 90% of the zinc may be extracted from the Hilton transition ore. A significant amount of iron is leached concurrently.

In a column trial simulating a heap leach situation over 80% zinc recovery was found to be achievable. 14% of the iron content was also extracted. No permeability problems, common in many heap leaching trials, were experienced.

10 The ferric iron in the bioleach liquor may be .o completely precipitated by limestone neutralisation to pH The zinc content of the liquor may then be recovered by two stages of solvent extraction using 12% di-2-ethylhexylphosphoric acid (O/A 2:1) followed by Do 0 two stages of strip using 200 g/L sulphuric acid (O/A 3.5:1).

o Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be 0 20 embodied in many other forms.

Claims (18)

1. A method of extracting one or more valuable metals from a zinc transition ore by biological leaching of the ore wherein the leaching may be heap, dump, or in-situ leaching.

2. A method according to claim 1, wherein the biological leaching includes the use of one or more microorgailisms selected from the group consisting of T. Ferrooxidans, L. Ferrooxidans, T. Thiooxidans, 10 heterotrophic organisms, and mixtures thereof.

3. A method according to claim 1 or 2 wherein the transition ore contains sphalerite.

4. A meLhod according to any one of claims 1 to 3, wherein the pH during leaching is controlled at from 1.23 15 to 3.

5. A method according to claim 4 wherein the pH during leaching is from 1.6

6. A method according to any one of the preceding claims wherein the temperature during leaching is from to 350C.

7. A method according to claim 7 wherein the temperature is 30 0 C.

8. A method according to any one of the preceding claims wherein, after initially bringing the pH to below 2.5, the pH is maintained at below pH 2.5 by acid produced in-situ by bacterial oxidation of sulphide.

9. A method according to any one of the preceding claims wherein the pH is lowered to below pH 1 prior to leaching. if l 19 A method according to Claim 8 comprising the step of treating the transition ore with sufficient acid to lower the pH of the leachate to below 2.5 prior to or during innoculation of the transition ore with one or more microorganisms.

11. A method according to any one of the preceding claims wherein the transition ore contains iron and the leach solution obtained from the biological leaching is reacted with lime or limestone to precipitate an iron product.

12. A method according to claim 11 wherein the iron product is precipitated as ferric iron Fe(OH)

13. A method according to any one of claims 11 or 12 wherein the pH is between 2.0 and 5.0 during precipitation of the iron product.

14. A method according to claim 13 wherein the pH is during precipitation.

15. A method according to any one of the preceding claims wherein iron, if any, is removed from tha leach solution obtained by biological leaching and then the zinc is extracted from the leach solution by solvent extraction.

16. A method according to claim 15 wherein the solvent Y extraction step is conducted using di-ethyl hexyl phosphoric acid as the solvent.

17. A method. according to claims 15 or 16 wherein the pH during solvent extraction is maintained at a pH of from 3.0 to

18. A method according to claim 17 wherein ammonia or sodium hydroxide is used to maintain the pH during solvent extraction.

19. A method of extracting zinc from a zinc containing transition ore by biological leaching which method is substantially in accordance with any one of the foregoing examples. DATED this 23rd Day of MAY, 1994 MOUNT ISA MINES LIMITED Attorney: IAN T. ERNST Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS 9*t S V, i ,~n A) ABSTRACT A method of extracting one or more valuable metals from a transition ore by biological leaching of the ore wherein the leaching may be heap or in-situ leaching. A method of extracting an iron product from a leach solution comprising the step of reacting the leach solution with lime or limestone to precipitate an iron product. 0rn~m o UUUU~L (o U o U 0 t o a o a a a. a a tll I U Li L i 1