Transcript Base metals recovery from zinc hydrometallurgical plant

(1) University of Lubumbashi
(2) The Free University of Brussels
D.R.Congo
Belgium
Base metals recovery from zinc
hydrometallurgical plant residues
by digestion method
R. B. Ngenda (1), P. K. Kongolo (1) and L. Segers (2)
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).
Table 1: Chemical composition of UZK residues
Ag,
Element
Assay
Zn, %
19.5
Cu, %
2.7
Ge,
Ga,
Cd, %
0.16
Pb,
Fe, %
ppm
ppm
ppm
157.1
475.4
447.8
Granulometric size of UZK residues
100 % is under 106 µm, 80 % is smaller than 38 µm.
%
32.79
2.43
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)
Experimental procedure
Digestion experiments were conducted by adding a given volume
of 48 % H2SO4 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.
Figure 2: Photograph of the roasting equipment
The roasting equipment consists of a vertical resistance electric furnace.
Figure 3: Photograph of the leaching installation
The leaching vessel was externally heated by thermostatically controlled water circulation.
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 % H2SO4) has been
identified from preliminary tests as best digestion reagent.
Therefore this solutions was used all over the test series.
Table 2: Main crystalline phases (identified by XRD)
Element
Occurrence
Formula
Quantity, %-wt
Zinc (Zn)
Franklinite
ZnFe2O4
80.4
Willemite
Zn2SiO4
3.9
Sphalerite
(Zn, Fe)S
1.3
Silicon
Quartz
SiO2
1.6
Calcium
Gypsum
CaSO4.2H2O
2.6
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 MeFe2O4 (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):
MeFe2O4 + 4 H2SO4  MeSO4 + Fe2(SO4)3 + 4 H2O
(Me = Zn, Cu, Cd, etc.)
Iron also is converted to the soluble sulphates during this process.
(1)
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 H2SO4/t residues, S/L = 0.9 [g/mL], 24 h.
Figure 5: Solubilisation of base metals from UZK residues in function of the
digestion time. Test conditions: Digestion (1 t H2SO4/t residues, S/L = 0.9 [g/mL]),
if dry sample (100°C, 24 h), leaching (water, 2 h, 40°C).
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 ZnSO4.H2O, rhomboclase
FeH(SO4)2.4H2O and hohmannite Fe(H2(H2O)4((SO4)2O).4H2O.
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,H3O)Fe3(SO4)2(OH)6 has been rarely identified in leaching residues
from materials which have been digested for more than 4 h.
Impact of sulfuric acid consumption
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).
A specific acid consumption of 1 t H2SO4 / 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.
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 FeSO4 → Fe2O3 + 2 SO2 + ½ O2
(2)
2 Fe2(SO4)3 → 2 Fe2O3 + 6 SO2 + 3 O2
(3)
Mikasaite Fe2(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.
Roasting experiments
Figure 7: Leaching recovery of base metals as function of the roast time. Operating
conditions: digestion (1 t H2SO4/t UZK residues, drying (100°C, 24 h), grinding,
roasting (750 ° C), leaching (water, 40°C, 2 h).
Figure 8: Leaching recovery of base metals as function of the roast temperature.
Operating conditions: digestion (1 t H2SO4/t UZK residues, 24 h),drying (100°C,
24 h), grinding, roasting (750 ° C, 2 h), leaching (water, 40°C, 2 h).
UZK Residues
(Zn, Cu, Cd, Fe, Pb, Ge, Ga, Ag)
H2SO4
Digestion at ambiant temperature
S/L = 1/1.1 [g/mL]
1 t H2SO4 / t residues, 24 h
H2SO4
Pasty product
H2SO4
Production
Drying : 100°C, 24 h
Dry cake
Grinding
SO2 gas
Air
Roasting : 770°C, 2 h
Roast product
Leaching : water 40°C, 2 h
Decantation / Filtration
New Residues
Solution
(Zn, Cu, Cd)
(Fe, Pb, Ge, Ga, Ag)
Figure 9: Proposed flow sheet for the treatment of UZK residues by digestion method
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 % H2SO4
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 SO2 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 (Fe2O3) 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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