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  4. One of the processes used for concentration of ores is Froth floatation process. This process is generally used for concentration of sulphide ores. Sometimes in this process we add NaCN as a depressant. NaCN is generally added in case of ZnS and PbS minerals. What is the purpose of addition of NaCN during the process of Froth floatation ?

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Q. One of the processes used for concentration of ores is Froth floatation process. This process is generally used for concentration of sulphide ores. Sometimes in this process we add NaCN as a depressant. NaCN is generally added in case of ZnS and PbS minerals. What is the purpose of addition of NaCN during the process of Froth floatation ?

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Solution:

$\text{NaCN} & + & \text{ZnS} & → & \left(\text{Na}\right)_{2}\left[\text{Zn} \left(\text{CN}\right)_{4}\right] \\ & & & & \text{Soluble complex}$
A layer of this zinc complex is formed on the surface of ZnS and due to this ZnS is prevented from the froth formation while PbS form froth. (i.e., NaCN is added as depressant for ZnS).