ENG326 Hydrometallurgy

Question 1

a) Compare and contrast the properties of "activated carbon" and a "strong base resin" for adsorbing gold from typical cyanide process liquors.

b) Describe their structure/functional groups and the mechanism by which gold is

loaded. Summarize the advantages/disadvantages of resins.

c) Draw a flowsheet of a typical carbon-in-pulp process which treats gold, consisting of 4 leaching and 6 adsorption stages, including the associated comminution, elution, regeneration and electrowinning sections of the full circuit, and briefly describe theoperation.

Question 2

a) Explain why the kinetic rate constant (k) is more important in a CIP plant than the equilibrium capacity of gold on carbon (K)?

b) State what factors affect the k value of carbon?

c) A gold leach solution was analysed and found to contain the following elements besides gold in cyanide elution at pH10: Na, Ag, Cu, Ni, Fe, Si, Ca, S, As, Cl.

i. List the exact (dominant) species in solution which contain these elements, and state which do you predict will absorb on carbon? Justify.

ii. Name 3 other chemicals that may exist in the solution which must be considered which can foul carbon.

Question 3

a) Using the data in your notes, calculate the overpotential for the reduction of Ni2+ ions at a current density of 300 A/m2 at a nickel cathode in a solution containing 1 M Ni2+. Assume that the high-field approximation to the Butler-Volmer equation can be used and that 2 x 10-3 A/m2 is the exchange current density for this reaction.

b) Calculate the potential of the cathode assuming initially that the only reaction is the reduction of nickel ions.

c) Calculate the current due to the reduction of H+ at the cathode potential derived in (b) for a solution pH = 3. You can assume that the exchange current density for hydrogen evolution of 1 x 10-3 A/m2 is proportional to the square root of the proton activity and that the high-field approximation can be used.

d) Calculate the current efficiency for the deposition of nickel at pH 3 and 300 A/m2

Question 4

The main reaction in zinc electrowinning is:

Zn2+ + 2 e- → Zn(s) (Cathode half cell Reaction)

H2O → ½ O2(g) + 2H+ + 2 e- (Anode half cell Reaction)

Zn2+ + H2O → Zn(s) + ½ O2(g) + 2H+ (Overall Reaction)

There is a second undesired reaction that negatively impacts the current efficiency:

2H+ + 2 e- → H2(g) (Cathode half cell Reaction)

H2O → ½ O2(g) + 2H+ + 2 e- (Anode half cell Reaction)

H2O → ½ O2(g) + H2(g) (Overall Reaction)

For this question the current efficiency is 90%. Note that P (Watts) =V(Volts) * I(Amps); V

= I*R(Ohms); Amps = C(Coulombs)/s; 96,485 C/mole- e-

; MZn = 65 g/mole

a) What is the ratio (in moles) of hydrogen gas to oxygen gas generated?

b) Calculate the potential required to drive the process at rate of 500 A knowing the anode reaction occurs at potentials above 1.23 V vs. SHE and the cathode reaction occurs at potentials below -0.76 V vs. SHE. The anode and cathode overpotentials are 0.7 and 0.1 V respectively. Also at this electrowinning rate, the loss in potential due to the electrolyte and hardware are 0.5 and 0.2 V respectively.

c) What is the specific energy consumption (in kWhr/kg-Zn) for this process?

d) What materials are the anodes and cathodes used in zinc electrowinning process typically made from?