1. Base metals recovery from zinc hydrometallurgical plant residues by digestion method R. B. Ngenda (1), P. K. Kongolo (1) and L. Segers (2) (1) University of Lubumbashi D.R.Congo (2) The Free University of Brussels Belgium

2. ElementZn, %Cu, %Cd, % Ag, ppm Ge, ppm Ga, ppm Fe, % Pb, % Assay19.52.70.16157.1475.4447.832.792.43 Table 1: Chemical composition of UZK residues Genesis of UZK residues The “Kolwezi Zinc Plant”, in French “Usines à Zinc de Kolwezi” (UZK), has produced about 910,000 dry metric tons of residues during the hydrometallurgical treatment of calcines from sulphide zinc concentrates (Roast-Leach-Electrowinning, RLE Process). Granulometric size of UZK residues 100 % is under 106 µm, 80 % is smaller than 38 µm.

3. Figure 1: Some views of the residues ponds (a) Crevasse in pond n° 4, (b) Storm of residues dust to the plant’s offices. (August 2008)

4. Experimental procedure Digestion experiments were conducted by adding a given volume of 48 % H 2 SO 4 solution to a given quantity of UZK residues, without any agitation or heating, allowing the digestion reaction to take place during some hours. The resulting dough was then dried overnight in oven prior to grinding. This product was then roasted and thereafter leached in water under agitation. After leaching, the leach pulp was filtered and the solid cake washed until a colorless filtrate was obtained. The filtrate was adjusted to 1000 mL while the solid was dried overnight and weighted thereafter. The solids and liquids from different process operations were analysed using the appropriate methods and apparatus.

5. Figure 2: Photograph of the roasting equipment The roasting equipment consists of a vertical resistance electric furnace.

6. Figure 3: Photograph of the leaching installation The leaching vessel was externally heated by thermostatically controlled water circulation.

7. Analytical equipment and reagents Different analyses have been performed with the following appropriate equipments: Crystalline phase identification on solid by X-ray diffraction (XRD) Semi quantitative analysis in solid by X-ray fluorescence (XRF) Chemical analysis of liquid by Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) Grain morphology study by Scanning Electron Microscope (SEM) Simultaneous thermogravimetric and differential thermal analyses, coupled with mass spectrometry. Granulometric analysis by sieving and laser diffraction. Half concentrated sulfuric acid solution (48 % H 2 SO 4 ) has been identified from preliminary tests as best digestion reagent. Therefore this solutions was used all over the test series.

8. ElementOccurrenceFormulaQuantity, %-wt Zinc (Zn)FrankliniteZnFe 2 O 4 80.4 WillemiteZn 2 SiO 4 3.9 Sphalerite(Zn, Fe)S1.3 SiliconQuartzSiO 2 1.6 CalciumGypsumCaSO 4.2H 2 O2.6 Table 2: Main crystalline phases (identified by XRD)

9. Sulphatation of zinc ferrites by sulfuric acid digestion Digestion process Metallic oxides which are present in UZK residues are essentially ferrites with the general formula MeFe 2 O 4 (Me = Zn, Cu, Cd, etc.). Digestion with sulfuric acid is conducted by allowing the mixture of acid with the material to stand for a long time, without agitation. The acid enters the matrix of ferrite to transform its structure by liberating the metals in the form of soluble sulphates according to the general reaction (1): MeFe 2 O 4 + 4 H 2 SO 4  MeSO 4 + Fe 2 (SO 4 ) 3 + 4 H 2 O (1) (Me = Zn, Cu, Cd, etc.) Iron also is converted to the soluble sulphates during this process.

10. Microscopic images of grain morphology for the raw material and the dried and ground digested one can be seen in figure 4. Grain attack by acid is clearly visible in the figures. The white areas are dominated by acid presence. There is also presence of iron sulphate precipitate. (a) (b) (c) Figure 4 : Morphology of UZK residues grains before (a) and after digestion (b), (c) a white zone of high sulfuric acid quantity. Digestion conditions: 1 t H 2 SO 4 /t residues, S/L = 0.9 [g/mL], 24 h.

11. Figure 5: Solubilisation of base metals from UZK residues in function of the digestion time. Test conditions: Digestion (1 t H 2 SO 4 /t residues, S/L = 0.9 [g/mL]), if dry sample (100°C, 24 h), leaching (water, 2 h, 40°C).

12. Impact of the digestion time and of drying XRD analyses performed on digested materials have confirmed sulphatation of zinc and iron into compounds like gunningite ZnSO 4.H 2 O, rhomboclase FeH(SO 4 ) 2.4H 2 O and hohmannite Fe(H 2 (H 2 O) 4 ((SO 4 ) 2 O).4H 2 O. Leaching in water of digestion cake at different times could solubilise zinc, copper and iron to approximately 70 % irrespective of the digestion time. The result is shown in figure 5. It was observed that metal leaching was less on wet samples compared to the same samples after they have been dried, suggesting that the sulphatation process still continues during sample drying in oven. After leaching, the residues mainly contain: franklinite (75 %), anglesite (15 %), quartz (7 %) and sphalerite (4 %). Jarosite (K,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 has been rarely identified in leaching residues from materials which have been digested for more than 4 h.

13. Figure 6: Solubilisation of base metals from UZK residues in function of sulfuric acid consumption. Test conditions: Digestion (S/L = 0.9 [g/mL]), leaching (water, 2 h, 40°C). Impact of sulfuric acid consumption

14. A specific acid consumption of 1 t H 2 SO 4 / t UZK residues seemed to be good for the practice. Sulphatation efficiency was high and the resulting product easy to manipulate. Leaching was performed to 79.5 % Zn, 87.4 % Cu and 82.8 % Fe.

15. Investigation on thermal decomposition Thermogravimetric, differential thermal analyses and mass spectrometry Iron conversion from the soluble sulphate compound to the non soluble oxide one occurs at high temperature according to reactions (2)(3) [7, 8]: 2 FeSO 4 → Fe 2 O 3 + 2 SO 2 + ½ O 2 (2) 2 Fe 2 (SO 4 ) 3 → 2 Fe 2 O 3 + 6 SO 2 + 3 O 2 (3) Mikasaite Fe 2 (SO4) 3 has been identified as the main iron compound in digested materials. Sulphates of other metals also undergo thermal conversion in a similar. Selective iron sulphate decomposition occurs between 650 ° C and 850 ° C.

16. Figure 7: Leaching recovery of base metals as function of the roast time. Operating conditions: digestion (1 t H 2 SO 4 /t UZK residues, drying (100°C, 24 h), grinding, roasting (750 ° C), leaching (water, 40°C, 2 h). Roasting experiments

17. Figure 8: Leaching recovery of base metals as function of the roast temperature. Operating conditions: digestion (1 t H 2 SO 4 /t UZK residues, 24 h),drying (100°C, 24 h), grinding, roasting (750 ° C, 2 h), leaching (water, 40°C, 2 h).

18. UZK Residues Digestion at ambiant temperature S/L = 1/1.1 [g/mL] 1 t H 2 SO 4 / t residues, 24 h Drying : 100°C, 24 h Grinding Roasting : 770°C, 2 h Leaching : water 40°C, 2 h Decantation / Filtration H 2 SO 4 Production SO 2 gas H 2 SO 4 Air New Residues Solution Pasty product Dry cake Roast product (Zn, Cu, Cd) (Fe, Pb, Ge, Ga, Ag) (Zn, Cu, Cd, Fe, Pb, Ge, Ga, Ag) H 2 SO 4 Figure 9: Proposed flow sheet for the treatment of UZK residues by digestion method

19. SUMMARY AND CONCLUSIONS The method used for the treatment of residues from the Zinc Hydro Plant of Kolwezi essentially consists of digestion of the materials with a 48 % H 2 SO 4 solution without agitation or heating. After digestion, drying and grinding of the obtained compact product, roasting was performed prior to leaching with water. Zinc, copper and cadmium were leached into solution, while iron preferentially remained in the leach residues. The SO 2 roast gas can be used for the manufacture of sulfuric acid. Leaching could be performed to 98.7 % Zn, 99.9 % Cu, almost 100 % Cd and only 6.4 % Fe under the best test conditions as given in the flow sheet. Iron preferentially remained as hematite (Fe 2 O 3 ) in the new leaching residues. The new leaching residues may be considered as Ge (800 ppm) and Ga (1660 ppm) concentrates containing most of the silver (Ag).

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