GB1604768A - Recovery of platinum group metal values - Google Patents

Description

(54) RECOVERY OF PLATINUM GROUP METAL VALUES (71) We, JOHNSON, MATTHEY & CO., LIMITED, a B-ritish Company, of 43 Hatton Garden, London ECIN 8EE, do herebgadeclare the invention, for which we pra that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the recovery of metal values; more particularly it relates to the recovery of precious metal values from mineral waste which hays been discarded after treatment.

In the case of the recovery of precious metal values from the mineral ore which bears them, the unrecovered residue which may amount to only part per million can represent a very considerable loss when it is realised that over a period of time millions of tons of ore are processed.

It is an object of the present invention to recover at least some of the unrecovered residue of metal value, particularly precious metal value after the ore has passed through all the usual stages of treatment.

According to the present invention a process for the recovery of platinum group metal values from treated mineral ore comprises the following steps:- (a) planting a green leaf-producing plant in a substrate which includes the said mineral ore; (b) allowing the said plant to grow in the substrate with access to light and water for a period of not less than seven days; (c) recovering part or all of the plant by harvesting; (d) ashing; (e) recovery of the metal values from the ash residue from step (d) by a pyrometallurgical or a chemical solution technique.

In In step (a) the mineral ore itself may constitute the substrate without further addition or subtraction. In this case the surface of a tip in a mining area can be used.

Alternatively, the role of the ore as a soil for plant growth can be improved by for example the addition of fertilizer or dilution with natural topsoil. The treated ore is preferably spread out over existing land so that in absorbing as large a fraction as possible of the metal value in the ore the plant roots need only grow to their natural depth. The treated ore may be extended with natural topsoil as a.diluent before spreading ar it may be ploughed into the existing topsoil after spreading. Suitable varieties for planting are the various grasses, the tomato, the bean and the corn plants.

Alternatively the substrate in step (a) may form an aqueous reservoir such as a pond or lake into which is discharged, with dilution, the aqueous effluent from a refinery or flotation plant. In this case the green leaf producing plant may be of the water surface floating variety, e.g. water lily, water hyacinth, duckweed, pondweed etc. or it may be of the reed or rush variety.

In step (b) the exact period for which the plant is allowed to grow in the substrate depends upon the degree to which further metal uptake is inhibited by the presence of metal which has already been absorbed. A practical minimum period Is seven days. The time allowed for growth also depends on the concentration of metal in the substrate. A high concentration of a deactivating metal such as platinum will indicate a fairly short growth period because uptake of metal will come to a stop fairly quickly.

In step (c) whether whole or only part of the plant is harvested depends upon the plant, the substrate and the metal being absorbed. Where grass has been planted it is frequently more economic to cut the grass rather than uproot it and allow further translocation of metal value from the grass root to blade to occur before cutting it again. In other cases particularly with platinum, a large quantity of metal is deposited in the root. This forms an appreciable proportion of the total metal which is recovered and it is therefore appropriate to uproot the whole plant for recovery purposes. An example of the latter would be recovery of platinum with the tomato plant.

In step (d) the plant is converted to ash by strong heating at a controlled rate in which dehydration and carbonisation are the main reactions. The mineral contents of the plant remain as residue.

The ash residue from step (d) has its metal value recovered preferably by a known pyrometallurgical or chemical solution technique. An example of the former would be smelting with a flux and base metal oxide collector and an example of the latter would be washing to remove water soluble salts followed by dissolution in an oxidising acid solution such as concentrated hydrochloric acid containing chlorine. Preclpitation or solvent extraction of individual metals can then be carried out by methods known in the art. In the case of platinum group metals known methods are described in British Patent Specification Nos.

1,495,931, 1,497,535 and 1,497,534.

EXAMPLES 1-7 These examples were carried out in a Prestcold (Registered Trade Mark) controlled environment growth room. The conditions were: illumination, 103 ft candles; temperature, 20TI"C (14 hour day), 15+1 C (night), relative humidity, 65%.

(i) Uptake of Platinum Applied as (NH4)2PtCI , Seeds were placed in rows in a 355x217 mm seed tray of acid washed Loch Aline sand for germination. The sand was moistened with demineralised water and the tray was transferred to the growth room. During the germination period the tray was covered with polyethylene film. When the first leaves had become established, the individual seedlings, selected for uniformity of growth, were transferred to 2 dm3 black plastic boxes. Half strength nutrient solutions (Epstein (1972) were added to six boxes each containing four plants. When the plants were well established the boxes were treated with full strength nutrient solutions containing the following concentrations of platinum: 0.05, 0.5, 2.5, 5.0, 10.0, 30.0 ppm.

After 14 days the plants were harvested. Wet weights of the plant tops and roots were taken. The samples were then dried and reweighed. After wet ashing with concentrated nitric acid, the mixtures were taken to dryness. 2 cm3 concentrated HCI and 5 cm3 lanthanum chloride solution (200 mg dm-3) were added and the samples were made up to 50 cm3 for atomic absorption analysis.

Standards were made up similarly. Results at the low end of the sensitivity scale were diluted and re-analysed using the carbon furnace. Calcium was assayed similarly in the presence of strontium chloride (5 mg dm-3).

(ii) The Uptake of Palladium Applied as NazPdCl4 The methods described above were used. When the plants were well established full nutrient solutions containing the following concentrations of palladium were added: 0.05, 0.5, 2.5, 5.0 ppm. The samples were analysed as before by atomic absorption techniques (well and Gidly 1966).

(iii) The Uptake of Rhodium Applied as Na RhCl6 The method was identical to that used for palladium.

The results of the growth and uptake experiments are shown in Tables 1--7.

Plants fed with platinum solutions showed normal healthy growth with 0.05 ppm platinum. At 0.5 and 2.5 ppm levels there was evidence of chlorosis and a slowing of growth. Little growth was apparent at 5.0 ppm, roots were yellow and stunted; at 30 ppm there was no growth, severe yellowing and stunting of roots, and necrotic spots on the lower leaves, For palladium treated plants, dropoff in yield occurs between the 0.5 and 2.5 ppm levels, the effect is similar to that found with platinum treated plants but less drastic.

At the concentrations investigated, rhodium had little effect on the growth of the plants.

TABLE 1 Yields and Analysis of Tomato Plants Treated With (NH4)2PtCI6 Tops Al B1 Cl Dl El Fl Applied Pt/ppm 0.05 0.50 2.50 5.00 10.00 30.00 Wetweightlg 63.15 58.91 21.44 7.27 3.53 2.10 Dry weight/g 4.060 3.630 1.473 0.752 0.433 0.265 Weight of water/g 59.09 55.28 19.97 6.52 3.10 1.83 Water present % 93.6 93.8 93.1 89.7 87.8 87.4 Total Pt,ug 6.0 23 32 10 19 40 Pt(drywt)/ppm 1.5 6.3 21.7 13.3 43.8 1.51 Total Ca/mg 105 92 43.5 12.4 7.8 4.3 Ca (dry wt)/X 2.59 2.53 2.34 1.65 1.80 1.62 Box Roots Al B1 Cl Dl El Fl Applied Pt/ppm 0.05 0.50 2.50 5.00 10.00 30.00 Wetweightlg 11.60 11.23 7.00 3.47 1.11 0.24 Dry weightlg 0.720 0.764 0.560 0.318 0.170 0.05 Weightofwater/g 10.88 10.47 6.46 3.15 0.94 0.19 Water present/% 93.8 93.2 92.3 90.8 84.7 79.2 Total Pt g 12 480 950 1425 1625 850 Pt (dry wt)/ppm 16.7 630 1760 4480 9560 17000 Total Ca/mg 6.5 5.9 4.6 2.1 1.3 0.43 Ca (dry wt)/% 0.90 0.772 0.852 0.660 0.765 0.860 TABLE 2 Yields and Analysis of Corn Plants Treated With (NH4)2PtCI3 Box Tops A2 B2 C2 D2 Applied Pt/ppm 0.05 0.50 2.50 5.00 Wet wt/g 7.76 10.436 4.446 4.557 Dry wt(g 0.473 0.762 0.344 0.346 Wtofwater/g 7.292 9.813 4.102 4.211 Water present/% 93.9 93.6 92.3 92.4 Total Pt g 2.5 3.0 4.5 8.5 Pt (dry wt)/ppm 5.3 4.5 13.1 24.6 Box Roots A2 B2 C2 D2 Applied Pt/ppm 0.05 0.50 2.50 5.00 Wet wt/g 3.137 3.716 2.003 3.056 Drywt/g 0.178 0.219 0.201 0.308 Wt of water/g 2.959 3.425 1.803 2.749 Water present/% 94.3 92.2 90.0 89.9 TotalPtg 25 75 300 1100 Pt (dry wt)/ppm 140 343 1496 3575 TABLE 3 Yields and Analysis of Bean Plants Treated With (NH4)2PtCI, Box Tops A3 B3 C3 D3 E3 F3 Applied Pt/ppm 0.05 0.50 2.50 5.00 10.00 30.00 Wetwt/g 24.16 15.08 12.03 13.22 7.72 3.61 Dry wt/g 2.214 1.279 1.386 1.578 1.986 0.511 Water present/% 90.8 91.5 88.5 88.1 87.2 85.9 Total Pt g 1 5 9 40 28 65 Pt (dry wt)/ppm 0.4 3.9 6.5 25.3 28.4 127 Total Ca/mg 55 23.5 21.8 20.5 11.8 4.6 Ca (dry wt)/% 2.48 1.84 1.57 1.30 1.19 0.90 TABLE 3 (cont.) Box Roots A3 B3 C3 D3 E3 F3 Applied Pt/ppm 0.05 0.50 2.50 5.00 10.00 30.00 Wet wt/g 8.53 5.40 2.71 2.38 1.63 1.21 Dry wt/g 0.527 0.301 0.336 0.303 0.221 0.121 Water present/% 93.8 94.4 87.6 87.3 86.4 89.9 Total Pt g 40 340 880 1650 1400 3100 Pt (dry wt)/ppm 76 1130 2620 5440 6330 25560 Total Ca/mg 2.4 1.7 1.6 1.5 0.9 1.0 Ca (dry wt)/% 0.456 0.550 0.460 0.48 0.41 0.85 TABLE 4 Yields and Analysis of Tomato Plants Treated With Na2PdCI4 Box Tops G1 Hl J1 K1 Applied Pd/ppm 0.05 0.50 2.50 5.00 Wetwtlg 36.88 38.50 8.0 7.340 Drywt/g 2.325 2.317 0.624 0.572 Water present/% 93.7 94.0 92.2 92.2 Total Pd g 1.5 2.3 2.4 3.2 Pd (dry wt)/ppm 0.64 1.01 3.8 5.5 Box Roots G1 HI J1 Applied Pdlppm 0.05 0.50 2.50 5.00 Wetwt/g 8.515 9.897 1/765 2.142 Drywtlg 0.607 0.670 0.210 0.184 Water present 92.9 93.2 88.2 91.5 Total Pd g 40.0 149 525 3170 Pd (dry wt)/ppm 65.9 222 2504 17190 TABLE 5 Yields and Analysis of Corn Plants Treated With Na2PdCl4 G2 Tops H2 G2 Roots H2 Applied Pd/ppm 2.5 5.0 2.5 5.0 Wet wt/g 4.187 5.439 2.029 3.056 Drywt/g 0.306 0.404 0.190 0.297 Water present/% 92.7 92.6 90.7 90.3 Total Pd g 1.5 4.5 410 2150 Pd (dry wt)/ppm 4.9 11.1 2160 7237 TABLE 6 Yields and Analysis of Tomato Plants Treated With Na3RhC15 Box Tops L1 M1 N1 P1 Applied Rh/ppm 0.05 0.50 2.50 5.00 Wetwt/g 12.80 34.08 36.85 37.19 Drywt/g 0.857 2.025 2.309 2.149 Water present/% 93.0 94.0 93.7 94.2 Total Rh ,ug 0.75 10;;5 46.0 65.0 Rh (dry wt)/ppm 0.87 5.2 19.9 30.3 Box Roots L1 M1 N1 P1 Applied Rh/ppm 0.05 0.50 2.50 5.00 Wet wt/g 4.944 6.554 8.651 7.676 Dry wt/g 0.259 0.418 0.532 0.500 Water present/% 94.8 93.6 93.9 93.5 Total Rh g 10.0 142 450 385 Rh (dry wt)/ppm 38.7 339 847 771 TABLE 7 Yields and Analysis of Corn Plants Treated With Na3RhCIs Tops Roots L2 M2 L2 M2 Applied Rh/ppm 10.0 30.0 10.0 30.0 Wetwt/g 13.44 8.134 4.406 3.632 Dry wt/g 0.783 0.540 0.261 0.247 Water present/% 94.2 93.4 94.1 93.2 Total Rh ,ug 22.3 45 45 108 Rh (dry wt)/ppm 28.4 83.3 173 438 The percentage of water in the plant, the yield and the calcium content decreases with increasing concentrations of platinum applied.Tables 1--3 show that this behaviour is observed for tomato, bean and corn plants treated with (NH4)2PtCl6. In addition, the platinum taken up by the plants increases as the concentration of applied platinum is increased. In all three plants platinum is translocated to the leaves. The roots appear to extract platinum from solution, and the high levels shown by tomato and bean roots at 5.0, 10.0 and 30.0 ppm Pt, when little or no root growth takes place, could be explained by the precipitation of platinum species in the free spaces of the roots. This precipitation would cause their yellow colour. The percentages of the total platinum removed by the tomato plants was: A, 14%; B, 29%; C, 14%; D, 5%; E, 3%; F, 1%. They are thus reasonably efficient at the removal of platinum from dilute solution.

Tables 4 and 5 show the results from the treatment of tomato and corn plants with Na2 PdCl4. The limited results indicate that there is some transfer to the leaves, and that like platinum, palladium is deposited in the roots.

Results for rhodium, applied as Na3RhCI, are shown in Tables 6 and 7. Tomato plants are not affected by rhodium at the concentrations studied. There is some transfer of metal to the leavens, but much'less metal is deposited in the roots than with either platinum or palladium.

Platinum At the levels tested all three plants behaved similarly to platinum stress. With such small samples for analysis yield alone cannot be taken as an indication of normal growth. A better indicator of this is the percentage of water present in the plant tissues, and also the levels of calcium within the plant.

Tolerance to the metal decreases in the order corn > tomato > bean. Corn is particularly efficient at taking up and translocating platinum at the lowest level, 0.05 ppm. Metal taken up by the roots in each case accounts for nearly all the platinum taken up by all three species. Both bean and tomato accumulate higher levels of platinum than corn does. But the optimum uptake with healthy growth was observed for corn at the 5.00 ppm level.

Palladium Corn and tomato plants behaved similarly under palladium stress. Both suffering deleterious effects at the 2.5 and 5.0 ppm levels, although corn seems to be slightly less serious affected. Insignificant levels of metal are translocated in both species.

Rhodium At all levels of rhodium nutrition tested, no toxic symptoms were observed in either tomato or corn. On the contrary very healthy growth was observed. Again tomato appears to accumulate higher levels of this metal than does corn.

These results would suggest that corn is the more tolerant. Both species translocate much larger quantities of this metal than of either palladium or platinum. The corn plants at the higher nutritional levels translocated 20% of dry weight metal levels in roots to the tops, which is nearer 500/0 as a percentage of total plant metal distribution.

EXAMPLE 8 In an attempt to ascertain whether the important factor in toxicity is the platinum ion itself, the complex or simply a molecular size effect. Corn plants were grown as before with different platinum complexes added to the normal nutrient solutions. The species investigated were PtCl42-, PtCl62-, Pt(NH3)42+, Pt(NH3)64+. The levels chosen were those at which normal growth was observed for corn using PtC16- as the platinum source.

Results are presented in Table 8.

Normal water relations were maintained except with PtCl42- at the 5 ppm level.

Metal levels in the roots were broadly comparable with no obvious differences between the various ionic species. Low levels of platinum were translocated to the aenal parts except for Pt(NH3)42+ most especially at the 5 ppm level.

TABLE 8 Yields and Analyses of Corn Plants Treated With Platinum in Different Forms Tops Type of Applied Pt Wet Weight Dry Weight Weight of Water Total Pt Pt(Dry Wt) Box Complex ppm g g Water g Present % g ppm A K2Pt(II)Cl4 0.5 13.691 0.893 12.798 93.5 ~1 ~1 B (NH4)2Pt(IV)Cl6 0.5 14.723 0.810 13.914 94.5 10.5 13.0 C Pt(II)(NH3)4Cl2 0.5 19.925 1.127 18.798 94.3 21.3 18.9 D Pt(IV)(NH3)6Cl4 0.5 21.644 1.254 20.390 94.2 2.0 1.6 E K2Pt(II)Cl4 2.5 10.954 0.816 10.138 92.5 2.5 3.1 F (NH4)2Pt(IV)Cl6 2.5 16.952 1.073 15.879 93.7 ~1 ~1 G Pt(II)(NH3)Cl2 2.5 21.355 1.286 20.069 94.0 175 136 H Pt(IV)(NH3)6Cl4 2.5 18.193 1.087 17.107 94.0 4.1 3.8 Roots Type of Applied Pt Wet Weight Dry Weight Weight of Water Total Pt Pt(Dry Wt) Box Complex ppm g g Water g Present % g ppm A K2Pt(II)Cl4 0.5 4.054 0.286 3.768 92.9 175 612 B (NH4)2Pt(IV)Cl6 0.5 4.551 0.327 4.224 92.8 175 535 C Pt(II)(NH3)4Cl2 0.5 5.644 0.355 5.289 93.7 100 281 D Pt(IV)(NH3)6Cl4 0.5 6.689 0.438 6.251 93.5 150 343 E K2Pt(II)Cl4 2.5 4.041 0.4471 3.594 88.9 200 447 F (NH4)2Pt(IV)Cl6 2.5 7.026 0.473 6.533 93.3 175 370 G Pt(II)(NH3)Cl2 2.5 7.161 0.407 6.755 94.3 275 676 H Pt(IV)(NH3)6Cl4 2.5 5.971 0.391 5.580 93.4 350 895 Too much emphasis cannot be placed on one result.

EXAMPLE 9 Uptake of Platinum by Duckweed, Lemna Minor L The experiment was carried out under conditions identical to those stated previously. The experiment did not involve measurements of yields since the plants do not lend themselves to this kind of analysis. The leaves are #3 mm in diameter with practically invisible roots. The plants were grown in open litre boxes in one litre of half strength nutrient solutions with different levels of platinum applied.

Initially approximately 0.25 of the surface was covered with plants. After 10 days the surfaces of A and B were covered with plants and the experiment was terminated. At this stage healthy growth was observed in both A and B, but far less growth had taken place in C and a large proportion of plants were dead.

Results are presented in Table 9.

TABLE 9

Applied Pt Total Pt in Solution Total Pt Taken Up Box ppm g in 10 Days g A 0 0 0 B 0.5 500 175 C 2.5 2500 450 It appears that the toxicity threshold level for this species occurs between 0.5 and 2.5 ppm of platinum as PtCl:-, as was observed for corn, tomato and bean.

The successful large scale uptake of toxic metals using water plants has been demonstrated by Wolverton and McDonald in New Scientist 12 August 1976.

It would appear from these preliminary results that water plants will accumulate platinum from relatively dilute solutions of these metals. Because of rapidity of growth and ease of harvesting the roots, water plants, particularly eichhornia crass-pipes, would be most suitable for accumulating platinum group metals.

The invention includes metal values when refined by a process according to the invention.

WHAT WE CLAIM IS: 1. A process for the recovery of platinum group metal values from treated mineral ore comprising the following steps:- (a) planting a green leaf-producing plant in a substrate which includes the said mineral ore; (b) allowing the said plant to grow in the substrate with access to light and water for a period of not less than seven days; (c) recovering part or all of the plant by harvesting; (d) ashing; (e) recovery of the metal values from the ash residue from step (d) by a pyrometallurgical or a chemical solution technique.

2. A process according to Claim 1 wherein in step (a) the mineral ore itself constitutes the substrate per se.

3. A process according to Claim 1 wherein the substrate includes a fertilizer.

4. A process according to Claim 1 wherein the substrate is an aqueous reservoir containing the mineral ore.

5. A process according to any preceding claim wherein step (d) is carried out by heating the plant at a controlled rate so that dehydration and carbonisation are the main reactions.

6. A process according to Claim 5 wherein the metal value is recovered from the ash residue by smelting with a flux and a base metal oxide collector.

7. A process according to Claim 5 wherein the ash residue is washed to remove water soluble salts present and, thereafter, the remainder is treated by dissolution in an oxidising acid solution.

8. A process according to any preceding claim wherein the plants are supplied with platinum in the form of (NH4)2PtC16; PtCI:-; PtC16-; Pt(NH3)4+ or Pt(NH3)6+.

9. A process according to any one of Claims 1 to 7 wherein the plants are supplied with platinum in the form of (NH4)2PtC16; PtCI4-: PtCI6-; Pt(NH3)4-: Pt(NH3)6+ or Na2PdCI4; or Na3RhCI6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. Too much emphasis cannot be placed on one result. EXAMPLE 9 Uptake of Platinum by Duckweed, Lemna Minor L The experiment was carried out under conditions identical to those stated previously. The experiment did not involve measurements of yields since the plants do not lend themselves to this kind of analysis. The leaves are #3 mm in diameter with practically invisible roots. The plants were grown in open litre boxes in one litre of half strength nutrient solutions with different levels of platinum applied. Initially approximately 0.25 of the surface was covered with plants. After 10 days the surfaces of A and B were covered with plants and the experiment was terminated. At this stage healthy growth was observed in both A and B, but far less growth had taken place in C and a large proportion of plants were dead. Results are presented in Table 9. TABLE 9 Applied Pt Total Pt in Solution Total Pt Taken Up Box ppm g in 10 Days g A 0 0 0 B 0.5 500 175 C 2.5 2500 450 It appears that the toxicity threshold level for this species occurs between 0.5 and 2.5 ppm of platinum as PtCl:-, as was observed for corn, tomato and bean. The successful large scale uptake of toxic metals using water plants has been demonstrated by Wolverton and McDonald in New Scientist 12 August 1976. It would appear from these preliminary results that water plants will accumulate platinum from relatively dilute solutions of these metals. Because of rapidity of growth and ease of harvesting the roots, water plants, particularly eichhornia crass-pipes, would be most suitable for accumulating platinum group metals. The invention includes metal values when refined by a process according to the invention. WHAT WE CLAIM IS:

1. A process for the recovery of platinum group metal values from treated mineral ore comprising the following steps:- (a) planting a green leaf-producing plant in a substrate which includes the said mineral ore; (b) allowing the said plant to grow in the substrate with access to light and water for a period of not less than seven days; (c) recovering part or all of the plant by harvesting; (d) ashing; (e) recovery of the metal values from the ash residue from step (d) by a pyrometallurgical or a chemical solution technique.

2. A process according to Claim 1 wherein in step (a) the mineral ore itself constitutes the substrate per se.

3. A process according to Claim 1 wherein the substrate includes a fertilizer.

4. A process according to Claim 1 wherein the substrate is an aqueous reservoir containing the mineral ore.

5. A process according to any preceding claim wherein step (d) is carried out by heating the plant at a controlled rate so that dehydration and carbonisation are the main reactions.

6. A process according to Claim 5 wherein the metal value is recovered from the ash residue by smelting with a flux and a base metal oxide collector.

7. A process according to Claim 5 wherein the ash residue is washed to remove water soluble salts present and, thereafter, the remainder is treated by dissolution in an oxidising acid solution.

8. A process according to any preceding claim wherein the plants are supplied with platinum in the form of (NH4)2PtC16; PtCI:-; PtC16-; Pt(NH3)4+ or Pt(NH3)6+.

9. A process according to any one of Claims 1 to 7 wherein the plants are supplied with platinum in the form of (NH4)2PtC16; PtCI4-: PtCI6-; Pt(NH3)4-: Pt(NH3)6+ or Na2PdCI4; or Na3RhCI6.

10. A process for the recovery of metal values from treated mineral ore

according to Claim I and substantially as hereinbefore described and exemplified.