Extraction Process For Pb and Zn from Galena (PbS) and Sphalerite (ZnS) Ores

In the world, usually Galena ore (PbS) is found together with Sphalerite ore (ZnS) in sulfide mineral. The Grade of PbS and ZnS in the ore between 2-8% for PbS and 8-16 % for ZnS. Generally, to increase the concentration of the metal in sulfide mineral is used The Froth Flotation.

Froth Flotation is Physical Chemistry methode to separate achieve mineral and impurities by use the mineral interface difference. The mineral that is very easy to absord the water is called by Hydrofillic, and the other is Hydrofobic. The Hydrofillic particles will be in the pulp, and the others will be at the air bulb and flow to the atmospheric surface. Usually, we use the reagents to make the interface particles become hydrofillic or hydrofobic. The reagents that we use in flotation are: collector, frother and modifier such as activator, pH regulator, depresant dan dispersant.

Figure 1. Scheme of froth flotation in Denver Flotation Cell

Below is the function for the reagents :
1. Collector
Collector is the reagent that make mineral surface become hydrophobic. Usually, colector is heteropolar organic mineral, content polar and non-polar side. Non-polar side is hydrofobic and will be at the air bulb, and polar side will be at specific solid particles than the solid particles will go to atmospheric surface.

2. Frother
When the surface of the specific solid particle become hydrophobic, that particle must converge with the air bulb from aeration. But the problem is the air bulb will be broke by hit with solid particle, cell, and the other air bulb. So, to make the air bulb become the stable bulb, we must add frother to the pulp. Frother is the reagent that can decrease the surface tension of the bulb hence the bulb is stable. The Effective Frother usually content minimum 5 atoms of carbon in the main molecule.

Table 1. The Frothers that often be used

3. Modifier
Modifier such as : activator, depressant, dispersant and pH regulator often be added to the flotation process. Activator is the reagent that used to increase interaction between solid particle and collector. Depressant make chemist polar film on the surface of the solid particle hence more hydrofobic. Dispersant is used to avoid the agglomeration, hence the particle can interact with the collector and the air bulb well. pH regulator is used to control of pH in order that the hydrofobic system can work optimally.

In Froth flotation for galena and spalerite, the reagents that be used :
1. Xanthate as collector
2. Pine Oil as frother
3. CaO as pH Modifier
4. CuSO4 as Activator for Pb
5. ZnSO4 as Activator for Zn
Generally, Flotation process for PbS and ZnS on industry scale is done continuously. Flowsheet of the process can be seen in figure 2.

Figure 2. The Flowsheet of the flotation process

Based on flowsheet in figure 2, we can see that we get 4 kinds of concentrates:
1. Cons. PbS
2. Cons. Mix PbS and ZnS
3. Cons. ZnS
4. Cons. Sulfur
All these concentrates will be processed to get the end product.. Cons. Sulfur is often processed by oxydation become H2SO4, the reaction:

S + O2 = SO2
SO2 + 1/2 O2 + H2O = H2SO4

Sulfur can be processed to the other product too, for example: fertilize, soap, medicine etc.

After concentration be done, then extraction process for Pb and Zn from the concentrates. We can used pyro ore hydro route, but we will discuss about pyrometallurgy route. Generally, in pyrometallurgy route, the concentrate converted to pellet. The aim of this process is avoid many dust in roasting and smelting process.

Figure 3. Extraction process flowsheet of Zn and Pb from consentrate ZnS

The same process is also done for concentrate PbS and mix PbS-ZnS. The dominant reactions of roasting and smelting process are :

ZnS + 3/2 O2 = ZnO + SO2
PbS + 3/2 O2 = PbO + SO2
C + 1/2 O2 = CO
ZnO + PbO + 2CO = Zn + Pb + 2CO2

The dominant reactions of electrowinning process for crude Pb are :

Pb + H2SO4 + 1/2 O2 = PbSO4 + H2O
Pb2+ + 2e = Pb
2H2O = 4 H+ + O2 + 4e
2H2O + 2e = H2 + 2OH-
2H+ + SO42- = H2SO4

Actually the metals that more achieve such as Cadmium (Cd), Antimony, Bismut etc that we say as impurities can be extract in this process with a little additional treatment.

Extraction process of Pb and Zn from their ore is not availabe yet in Indonesia. But may be in the next year will be build by PT. Dairy Prima Mineral (One of Antam’s subsidiary at North Sumatra). One of the established process in China is Zhongjin Lingnan Nonfemet Co. Ltd