PRELIMINARY STAGE 2013
Osei Tutu SHS
Yaa Asantewaah SHS
MfantsiMan Girls
QUESTION
Three methods are used to concentrate gold from its ore.
1. Gold can be concentrated from its ore by reacting the powdered ore with sodium cyanide solution in the presence of oxygen.
What is the name or formula of the product formed?
ANSWER: Sodium dicyanoaurate(I), $Na[Au(CN)_2]$
SOLUTION:
In cyanidation, gold reacts with $NaCN$ and $O_2$ to form the soluble complex $Na[Au(CN)_2]$, used to extract gold from low-grade ore.
2. Gold can also be concentrated by treating the powdered ore with mercury.
What is the name of the product formed?
ANSWER: Gold amalgam
SOLUTION:
Mercury forms an amalgam with gold, separating it from other ore material (an environmentally hazardous small-scale mining method).
3. Gold ore is sometimes roasted before the concentration process.
Why is this done?
ANSWER: To remove sulfur and arsenic (present as sulfides/arsenides) by oxidizing them to gaseous oxides.
SOLUTION:
These impurities would otherwise interfere with gold extraction, so roasting drives them off as gases.
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PRACTICE QUESTIONS
1. QUESTION: In cyanidation, gold dissolves in sodium cyanide solution only when a certain gas is present. Name the gas.
ANSWER: Oxygen (from air)
SOLUTION:
$4Au+8NaCN+O_2+2H_2O\rightarrow4Na[Au(CN)_2]+4NaOH$
Oxygen oxidises the gold so that it can form the soluble cyanide complex.
2. QUESTION: Gold is recovered from sodium dicyanoaurate(I) solution by adding a more reactive metal powder. Name the metal commonly used.
ANSWER: Zinc
SOLUTION:
$2Na[Au(CN)_2]+Zn\rightarrow Na_2[Zn(CN)_4]+2Au$
Zinc is more reactive than gold, so it displaces the gold from the complex.
3. QUESTION: In one method, gold in cyanide solution is adsorbed onto a porous form of carbon added to the ore slurry. Name this form of carbon.
ANSWER: Activated carbon (charcoal)
SOLUTION:
This is the carbon-in-pulp process; the gold is later stripped from the carbon.
4. QUESTION: Name the very poisonous gas that forms if acid is added to the cyanide solution used in gold extraction.
ANSWER: Hydrogen cyanide (HCN)
SOLUTION:
$CN^-+H^+\rightarrow HCN$
Cyanide solutions are therefore kept alkaline.
5. QUESTION: Why is gold often found uncombined (native) in nature?
ANSWER: Gold is very unreactive, so it does not combine readily with oxygen, water or other elements.
SOLUTION:
Gold lies below hydrogen in the reactivity series and resists oxidation.
6. QUESTION: What is the purity of pure gold expressed in carats?
ANSWER: 24 carats or $2.4\times10^{1}$ carats
SOLUTION:
An alloy of 18 carats is eighteen parts gold in twenty-four by mass.