Question 1Level 2 · ExamSingle correct
The ore whose name, chemical formula and usual method of concentration are all correctly given is ______. Exactly one option is correct.
AMagnetite, Fe2O3, magnetic separation
BCalamine, ZnS, froth flotation
CCuprite, Cu2S, froth flotation
DCopper pyrites, CuFeS2, froth flotation
Show answer and solution
Answer: D
A: Magnetite is Fe3O4 (Fe2O3 is haematite). Wrong.
B: Calamine is ZnCO3; ZnS is zinc blende. Wrong.
C: Cuprite is Cu2O; Cu2S is copper glance. Wrong.
D: Copper pyrites is CuFeS2, a sulphide ore, and sulphide ores are concentrated by froth flotation. Correct.
Answer: D.
Difficulty: HardQuestion 2Level 3 · AdvancedSingle correct
A finely ground sulphide ore contains PbS and ZnS together with siliceous gangue. It is concentrated by froth flotation in water containing pine oil, a xanthate, cresol and a controlled amount of NaCN, with air blown through. The correct statement about this process is ______. Exactly one option is correct.
ANaCN forms a complex on the surface of the ZnS particles and keeps them from coming up with the froth, while PbS comes up with the froth.
BNaCN forms a complex on the surface of the PbS particles and keeps them from coming up with the froth, while ZnS comes up with the froth.
CCresol acts as the collector for the sulphides, while pine oil depresses the silica gangue.
DThe silica gangue particles are preferentially wetted by the oil and rise with the froth, while the sulphides sink.
Show answer and solution
Answer: A
A: NaCN is a depressant. It forms a cyanide complex on the surface of the ZnS particles, so ZnS stays wetted by water and does not come up with the froth, while PbS comes up with the froth. This separates the two sulphides. Correct.
B: NaCN does not act on PbS this way; it is ZnS that is depressed. Wrong.
C: Pine oil and xanthates are collectors, which make the mineral particles non-wettable by water; cresol is a froth stabiliser. Neither depresses the gangue. Wrong.
D: Silica gangue is wetted by water and sinks; it is the sulphide particles, wetted by the oil, that rise with the froth. Wrong.
Answer: A.
Difficulty: Very hardQuestion 3Level 3 · AdvancedSingle correct
Bauxite containing SiO2, Fe2O3 and TiO2 as impurities is digested with concentrated NaOH(aq) at 473-523 K and 35-36 bar. The correct statement about this leaching step and the steps that follow it is ______. Exactly one option is correct.
AFe2O3 dissolves as a sodium salt and is later removed when CO2 is passed into the solution.
BSiO2 remains undissolved with Fe2O3 and TiO2, so no silicon enters the solution.
CThe filtered aluminate solution is neutralised with CO2 and seeded; hydrated Al2O3 precipitates while sodium silicate stays in solution, and the precipitate is heated to about 1470 K to give pure Al2O3.
DPassing CO2 precipitates hydrated silica, while aluminium stays in solution as Na[Al(OH)4], from which Al2O3 is obtained by evaporating the solution.
Show answer and solution
Answer: C
Digestion: Al2O3(s)+2NaOH(aq)+3H2O(l)→2Na[Al(OH)4](aq). SiO2 also dissolves (as sodium silicate). Fe2O3 and TiO2 remain undissolved and are left behind and filtered off.
The aluminate solution is neutralised with CO2 and seeded with freshly prepared hydrated Al2O3; hydrated Al2O3 precipitates, while sodium silicate stays in solution. The precipitate is filtered, dried and heated to about 1470 K to give pure Al2O3.
A: Fe2O3 does not dissolve. Wrong.
B: SiO2 does dissolve as silicate. Wrong.
C: Correct, as above.
D: CO2 precipitates the hydrated alumina, not silica; silicate stays in solution. Wrong.
Answer: C.
Difficulty: Very hard