exm357ie20082 - “W Murdoch EXaminations Semester 2 2008 U...

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Unformatted text preview: “W Murdoch EXaminations - Semester 2, 2008 U N I V E R S I T Y EXAM PAPER TO BE COLLECTED AND RETURNED TO EXAMS OFFICE Al-T ER EXAM / Unit EXM357 HYDROMETALLURGY ExamType : Internal & External Reading time : 10 minutes (No note taking allowed) Exam Duration : 3 hours INSTRUCTIONS Answer 4 (four) questions. See pages 3, 5, and 8 for parts of answers to be written on the question paper. Ensure that you include the question paper and other graph papers with your name and student number, inside your answer book. See page 2 for useful formulae. Family Name: Given Name: Student No: Signature: EXAMINATION AIDS ALLOWED CLOSED BOOK EXAM Provided by the University ANSWER BOOKLET GRAPH PAPER (A4 1MM) Provided by the Candidate CALCULATOR (NO QWERTY KEYBOARD) Question 1 (25 marks] (a) (b) (C) Calculate the time required to completely dissolve a gold particle of 20 pm diameter by chlorine gas in a solution of 1 mol/L HCI to produce a solution of AuCl4'. The mass transfer coefficient of chlorine is 0.012 cm/s, and the dissolved chlorine concentration is 45 mg/L. The density of gold is 19.3 g cm'3. Atomic masses: CI = 35.45, Au = 197.0. [10 marks] (i) Calculate the concentration ratio of Ni2+ICu2+ which can exist at equilibrium with NiS(s) and CuS(s). The equilibrium constants for the dissolution of these two sulfides at 25°C are: NiS(s) = Ni2+ + 32" K = 10-21-9 CuS(s) = Cu2+ + s?— K = 10-364. [3 marks] (ii) A leach solution in a reaction vessel contains 12 g/L Ni(ll) and 0.52 glL Cu(l|). H28 gas is mixed with the solution and the partial pressure of H28 is maintained at 50.65 kPa at 25°C. Assuming that all the copper is precipitated as CuS, calculate the maximum pH below which the solution should be maintained in order to prevent the loss of MS due to reaction with residual H28. H28(g) = 2H* + 82' K = 10-209 Atomic masses: Ni = 58.69, Cu = 63.55. [7 marks] The rate of dissolution of goethite to Fe2(SO4)3 at 90°C in 1 mol/L H2804 is 0.205 gls Fe(lll) at a certain pulp density. Calculate the rate of addition of H2804 in kg/h required to maintain the acid concentration of 1 moi/L unchanged during the leaching of goethite at the desired pulp density. (Atomic masses: Fe = 55.85, 0 = 16.00, H = 1.008, 8 = 32.07) [5 marks] Question 2 [25 marks] The leaching of a batch of zinc-ferrite (ZnFe2O4) in 50 glL H2804 at 95°C yielded the results listed in Table 1 (see page 3 of this question paper). (a) Show graphically or othenNise that the initial dissolution rate of zinc-ferrite in 50 g/L H2804 during the first 3 hours is controlled by the diffusion of reactants or products through a solid layer. [6 marks] (b) The rate of dissolution of zinc ferrite in 50 g/L H2804 over a period of 5 h is given by the equation: dXIdt = — 0.017 t3 + 0.185 t2 — 0.658 t + 0.847 (X = fraction dissolved after time t). Calculate the overall leaching efficiency in a two stage ideal CSTR which has a slurry capacity of 50 t in each stage. Zinc ferrite is treated at a flow rate of 10 t/h solids at a pulp density of 40% (WM). [10 marks] (0) At what flow rate of solids would it be possible to achieve the same overall efficiency as in part (b) using a single stage CSTR of a slurry capacity of 50 t? [5 marks] qd) Propose two methods which can be used to separate zinc from iron after zinc ‘- ferrite leaching. [4 marks] Some useful formulae 1—(1—X)1/3 __ E1 = t , pR z' 1-3(1-X)2’3 +2(1-X)= 6bc?t =1 , pR r X 234323 V dt uestion 2 Table 1. Dissolution of zinc ferrite in 50 g/L H2804 at 95°C Name : Dissolution % Question 3 (25 marks] (a) The results shown in the following figure were obtained in an experiment carried out in the laboratory to extract Zn(ll) and Cu(ll) from a sulfate solution using D2EHPA (HA dissolved in kerosene). 02n(u) DCu(ll) 100 CD 0 O) O 40 % Extarction of M(||) 20 (i) What is the value of pH1/2 for Zn(ll)? [1 mark] (ii) Calculate the distribution coefficient for Zn(ll) at pH 2.0 and 3.0. [4 marks] (iii) Use your answer in part (a)(ii) of this question to predict the stoichiometry for extraction of Zn(ll) using D2EHPA and write a balanced chemical equation. [4 marks] (b) (i) Briefly explain the role of a redox mediator with reference to the leaching of uranium at Olympic Dam Operations. [2 marks] (ii) A single stage pilot scale solvent extraction unit extracts 80% of uranium from a feed solution containing 0.5 g/L uranium using Alamine 336 (R3N) dissolved in an organic solvent. The aqueous to organic flow rate ratio is 4:1 at pH 1.5. The organic phase used for this extraction contains 0.7 g/L uranium. Calculate the concentration of uranium in the loaded organic phase. [5 marks] (iii) Briefly explain, with reasons, the effect of having a high concentration of sulfuric acid or sodium chloride in the uranium leach liquor upon the uranium content in the loaded organic phase. [3 marks] [6 marks] The loaded organic phase in part (b)(ii) of this question is stripped at an organic to aqueous flow rate ratio of 0.321. Use the equilibrium data produces the stripped organic with 0.7 g/L uranium in a counter-current the stripping isotherm and predict the number of stages which operation. from the laboratory shake-out tests shown in Table 2 (page 5) to draw (iv) Name: uestion 3 Uranium in Uranium in aqueous phase organic phase Table 2. Data for uranium stripping -IL mam"fin“E.Em"____mm._u_mmmmmm___mn_mmmmmm Question 4125 marks] A commercial copper electrowinning cell contains 60 cathodes each with immersed dimensions of 1m x 1m. The distance between each cathode and anode is 50 mm. The cell is operated at a current density of 300 A/m2 at the cathode and the plating period between stripping cycles is 5 days. The current efficiency is 90% with most of the loss in efficiency due to the presence of 6 g/L of iron in the electrolyte which also contained 0.5 moi/L copper(ll) ions and 1 mol/L sulfuric acid. The overpotential at the cathode was measured as -100 mV and at the anode as 800 mV. The conductivity of the electrolyte was measured as 0.5 Slcm (S = Ohm"). (a) Calculate the total current in the cell. [5 marks] (b) Calculate the total mass of copper (in kg) produced in the cell during each plating cycle. [5 marks] (c) Calculate the cell voltage. [5 marks] (d) Calculate the energy consumption in kWh/kg metal. [5 marks] (e) Write the electrode reactions at the cathode and anode responsible for the loss in current efficiency. [5 marks] F = 96500 A s / moi, Atomic mass Cu = 63.5, E°(Cu2+/Cu) = 0.34 v, E° (H20/02) = 1.23 V. Question 5 (25 marks] The data shown on page 8 were obtained for the adsorption of gold from a batch of pulp onto activated carbon. (a) Fill in the blank cells (column of loading) of Table 3 (see page 8). ' [5 marks] (b) Show graphically or othewvise that this data is consistent with a rate equation of the form -d[Au]sIdt = k[Au]s where [Au]s is the concentration of gold in the solution at any time t and k is a rate constant. Estimate a value for k. [10 marks] (c) How would “k" be expected to vary with the mass of activated carbon and with the volume of the pulp used in the batch test? [4 marks] (d) The actual concentration of gold in the solution after 24 hours was found to be 0.2 mg/L. Compare this value with that calculated using the above rate equation and rate constant and suggest reasons for the large difference. [6 marks] Name: 5 uestion Table 3 s s a m . n O b r a C mg... .n____-_____m___ Egg“. ____ _au=....._____ End of Question Paper ...
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