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EXTRACTIVE METALLURGY OFEXTRACTIVE METALLURGY OF GOLDGOLD

66. Recovery from solution. Recovery from solution

Fathi HabashiFathi Habashi

Laval University, Quebec City, CanadaLaval University, Quebec City, Canada

Fathi.Habashi@arul.ulaval.caFathi.Habashi@arul.ulaval.ca

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Precipitation by zinc

This method is known by the name of its

inventors Merrill-Crowe.

The clear leach solution is first de-aerated, then

zinc dust added to precipitate gold and silver.

De-aeration is essential to prevent the re-

dissolution of gold.

Precipitation by metallic zinc is an oxidation -

reduction process that takes place as a result ofdisplacing the equilibrium of the complex ion.

2[Au(CN)2] + Zn 2Au + [Zn(CN)4]

2

[Au(CN)2] Au+ + 2CN

Zn Zn2+ + 2e-

Au+ + e- Au

Zn2+ + 4CN [Zn(CN)4]2

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Analysis of typical precipitates from a Merrill-Crowe process

S. Africa,

%USA, %

Au 30-40 14.4

Ag 3-6 0.35Cu 1-4 0.9

Zn 5-25 9.4Pb 5-15 2.1

Fe ---- -----

S ---- 4.6SiO2 ---- 25.7

Directly smelted with fluxes to producebullion. This usually applies to high gradeores containing 10-25 g/t gold

Acid leaching to remove excess zinc, calcination tooxidize other impurities, then smelting with fluxesto produce bullion. This usually applies to lowgrade ores containing ~ 5 g/t gold

Acid leaching followed by retorting to removemercury if present, then smelting with fluxes to

produce bullion

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Acid leaching must be conducted under a

well ventilated hood since toxic and

explosive gases such as hydrogen sulfide,

hydrogen, and arsine are evolved.

Calcination of the solids is usually

conducted on stainless steel trays

mounted in an electric furnace

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Each tray contains approximately 60 kg of moistgold slime and the furnace can accommodatesix to twelve trays.

Calcination takes place at 600-700oC for aperiod of about 16 hours.

After calcinations, the gold powder is mixed witha flux, mainly composed of borax and silica.

Smelting is essential to produce good gradebullion that can be accurately sampled and

assayed for its gold and silver content, and canbe easily transported to the refinery.

Smelting is conducted in a three electrode submergedarc tilting furnace. The gold powder and flux isadded portion wise, 100 kg at a time.

The smelting cycle occupies about three hours,comprising one hour for charging the furnace, oneand half hours for fusion, and half an hour for slagremoval and pouring.

Smelting temperatures ranges from 1200 to 1400oC.

When fusion is complete, the three carbon electrodesare withdrawn and the furnace tilted from the verticaltowards the horizontal position to pour the moltencontents through the discharge spout.

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The gold bullion is poured into standard

moulds which are designed to contain bars of

31 kg. Once the gold has solidified, the bars

are quenched in water and cleaned by hand

scrubbing.

The bars are then sampled by hand-drilling.

The arc furnace is normally operated withelectrodes set at 600 amperes and the life of thecarbon electrodes ranges from 6 to 10 fusions.

The average life of the furnace liner is 35 fusionsbut this figure can be extended if repairs aremade between fusions.

A ventilation hood is installed above the furnaceand all fumes and dust emanating from the

furnace are withdrawn though a fan and passedinto a filter.

All fine particles trapped in the filter are collectedand returned to the ensuing furnace charge.

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The resulting slag from smelting is usually returned tothe mill circuit for recovering any gold it may contain.

Fire bricks and furnace liners, when replaced, arecrushed, passed over a shaking table to recover goldbeads, and then also sent to the mill.

Floor sweepings, dust from the smelt-house walls andthe deposit in the ventilation filters are handled in thesame way.

In general, all material leaving the smelt-house isreturned to the milling circuit for gold recovery and final

disposal.

Processing after enrichment by activated charcoal

The enriched solution obtained after

elution of the charcoal may then be treated

by two methods depending on the

concentration of gold in solution:

Electrowinning

Precipitation by zinc

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Electrowinning

This method is applied when the solutionis relatively rich and may be conducted inthe following ways:

Deposition on steel wool cathodesfollowed by smelting to prepare a bullion.

Deposition on stainless steel woolcathodes followed by reversing the polarity

and electroplating the gold, then melting toproduce a bullion.

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Precipitation by zinc

This method applies for dilute solutions

and the procedure followed is the same as

the one previously mentioned.

Enrichment by ion exchange followed byelectrowinning

This method is used only in one plant atMuruntau Mine in Uzbekistan.

The process is rather complex and costlybecause of high consumption of reagents.

If weak base resin is used, the pH of thesolution must be adjusted to an

undesirably low levels, and if strong baseresin is used it is difficult to strip theloaded resin.

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Enrichment by solvent extraction followed by

precipitatinon

Extraction of gold cyanide complex by

organic solvents such as alkyl phosphorus

esters and amines.

No industrial development followed.