2019 Final Exam

THE UNIVERSITY OF BRITISH COLUMBIA Department of Materials Engineering MTRL 358 – Hydrometallurgy I Final Examination 2019 [Total marks = 54] Total time: 2.5 hours Answer all questions • This examination has 11 pages (including supplementary material). • Formulas and data are included on pages 9-11. • Marks for each question are shown in square brackets. 1. [6 marks] Briefly answer the following questions: (a) What are the two purposes of solvent extraction? Purification and upgrading of the PLS (upgrading means to increase the concentration of the desired species). (b) Indicate whether the following sentence is true or false: Leaching is the process of dissolving a solid, in whole or in part, into an aqueous solution. TRUE (c) Name two hydrometallurgical process steps or operations in which the principle of counter-current flows are applied. Any two of counter-current leaching, counter-current decantation and counter-current solvent extraction (just solvent extraction is acceptable). (d) What happens at the cathode in an electrochemical cell? Reduction , or electron transfer to effect reduction. (e) What is the name for the aqueous stream exiting a solvent extraction loading process? Raffinate This exam is closed book and closed notes. Read all questions carefully before beginning and work neatly. Calculators are allowed. Cell phones and other electronic devices MUST be left at the front of the exam room. Backpacks, bags, coats etc. MUST be left at the front of the exam room.

(f) Silver sulfide (Ag 2 S) is highly insoluble in water (K sp = 6 x 10 -51 ). True or false: because of its low solubility it can’t be leached. False 2. [5 marks] The mineral enargite has the formula Cu 3 AsS 4 . Write a balanced chemical reaction for leaching of enargite by oxygen in sulfuric acid to form cupric ion, sulfate and arsenic acid (H 3 AsO 4 ; a soluble weak acid – assume it is undissociated). Write the reaction in ionic and neutral forms. Include the phases in the reaction in neutral form. 2Cu 3 AsS 4 + 5H 2 O + 17.5O 2 = 6Cu +2 + 2H 3 AsO 4 + 8SO 4 2- + 4H + 2Cu 3 AsS 4 s + 5H 2 O l + 17.5O 2 g = 6CuSO 4 aq + 2H 3 AsO 4 aq + 2H 2 SO 4 aq 3. [5 marks] The mineral CuS reacts with cyanide according to the following reaction: 2CuS + 8CN - + 0.5O 2 + H 2 O = 2[Cu(CN) 3 ] 2- + 2SCN - + 2OH - An ore contains 0.11% copper (1.1 kg Cu/tonne of ore) in the form of CuS. The ore is processed at a rate of 1500 tonnes per hour. Assuming 75% of the mineral is leached, how much sodium cyanide would be consumed by this reaction in kg/h? 3820 kg NaCN/h 4. [8 marks] An Eh-pH diagram for the Fe-S-H 2 O system involving only pyrite is shown below. (Pyrite itself is not valuable, but it may contain gold.) Conditions are indicated in the diagram. Thermodynamic data are also shown in the table below. Species  G° f kJ Species  G° f kJ Fe +2 -78.7 H 2 S g -33.56 Fe(OH) 2 -492 HS - 11.44 Fe +3 -4.6 S 8 ( or S) 0 Fe(OH) 3 -704.63 SO 4 2- -744.55 FeS 2 -166 HSO 4 - -755.91 H 2 O l -237.15 (a) Is it possible to leach iron from FeS 2 at pH 8? Briefly explain your answer. No. At pH 8 reduction of FeS 2 forms Fe s and HS - . There is no soluble Fe species. Oxidation forms solid Fe(OH) 3 . Either way Fe is not dissolved.

(b) Write the balanced half reaction for the line numbered (1) in the diagram. Fe +2 + 2SO 4 2- + 16H + + 14e - = FeS 2 + 8H 2 O (c) Calculate the equation for the line in the form Eh = Slope x pH + Constant. Eh = -0.06762pH + 0.36125 (d) (i) Write the main reactants and products for the two half reactions for leaching of pyrite by ferric ion at pH 0.5. (The balanced half reactions are not necessary, just the main reactants and products for each.) Fe +3 /Fe +2 Fe +2 ,HSO 4 - /FeS 2 (ii) Estimate the Eh values (including the units) for these two half reactions from the diagram. Eh Fe+3/Fe+2 = 0.77 V; Eh Fe+2/HSO4-/FeS2 = 0.32 V (iii) Estimate the  E for the reaction.  E =0.45 V (iv) Is the reaction favourable? Briefly explain your answer. Yes;  E > 0 -1.0 -0.5 0.0 0.5 1.0 0 2 4 6 8 10 12 Eh V pH Eh-pH diagram for the iron-sulfur-water system at 25°C and 1 atm pressure, 0.2 m Fe solute activities and 0.5 m S solute activities. Fe +3 Fe +2 HSO 4 - SO 4 2- Fe(OH) 3 Fe +2 Fe Fe(OH) 2 H 2 S g HS - FeS 2 H+ H2 O2 H2O 1

5. [5 marks] Below is a simplified flowsheet for a zinc roast-leach-electrowin process. Zinc calcine is obtained by roasting a zinc sulfide concentrate which contains ZnS, FeS 2 and small amounts of other minerals. The calcine contains mainly ZnO and zinc ferrite (ZnFe 2 O 4 ). Electrowinning produces most of the sulfuric acid needed in leaching. The jarosite precipitation stage forms solid NH 4 Fe 3 (SO 4 ) 2 (OH) 6 , which is highly insoluble. Suitable conditions for jarosite precipitation are about 95°C and pH ~1.5. (This is not sufficient to leach the zinc ferrite.) A small fraction of the zinc calcine is used to adjust pH in jarosite precipitation. Jarosite will not redissolve in the hot acid leach. (a) In which step does Fe(III) enter solution? The hot acid leach (b) Is the jarosite precipitation step primarily a leaching process or a solution purification process? Briefly explain your answer. Solution purification. Fe(III) is removed from solution as jarosite which is separated from solution by solid-liquid separation. (Some leaching of calcine occurs in that it acts as a base for neutralization of some of the acid, but that is not the main point.) (c) Indicate which of the following statements are true: (i) This is an example of a co-current leaching process. (ii) A disadvantage of this process is that zinc in the form of zinc ferrite added with the calcine into jarosite precipitation is not recovered. (iii) In this process zinc present in the calcine as zinc ferrite is leached in the hot acid leach step. True (iv) In this process zinc present in the calcine as zinc ferrite is leached in the neutral leach step. (v) Good use of sulfuric acid in leaching is ensured by directing it back towards the neutral leach. True (vi) Zinc oxide is mainly dissolved in the neutral leach step. True (vii) Zinc oxide is mainly dissolved in the hot acid leach step.