Processing, Smelting & Refining of Gold

Table of contents

The gold processing starts from the ore and continues to the doré. Gold is popular for its value and beauty since ancient times. and beauty since ancient times. The chemical strength and color contribute to the prominence of this precious metal.

Gold processing is about preparing the gold ore for using it in different products. The vital gold ores come with gold in the Indigenous form. It is available in both endogenetic as well as exogenetic forms. Gold in the endogenetic form comes from the underground. On the other hand, the exogenetic form is available at the earth’s surface.

Alluvial gold is one of the most popular exogenetic ores. This type of gold is available in streambeds, riverbeds, etc. It always comes as elemental gold. Extremely fine particles make the alluvial gold.

Endogenetic gold ores contain elemental gold’s lode and vein deposits. It is available in compositions and quartzite of quartzite along with different types of iron sulfide minerals. It primarily includes pyrrhotite and pyrite. Native gold comes as the most common gold mineral. It contributes to around 80% of the metal in the exterior of the earth.

Ore Processing

Experts extract gold and silver from ores at the processing plant. Ore processing comprises the following steps:

  • The first step is about crushing as well as grinding the gold ore.
  • Experts add process water to make a slurry (a semi-liquid mixture).
  • Then, experts add lime to the ore, whereas they mix cyanide solution with the slurry to filter the precious metals into the solution.
  • Experts add carbon to attach it with the precious metals and pull them out from the slurry.
  • Acid washing and circular movement of a caustic cyanide solution remove gold and silver from the carbon.
  • Electro-winning is necessary to precipitate the precious metals.
  • Next, metal products are smelted into doré bullion bars.
  • Experts pump the unproductive slurry to the tailings storage facility.

Gold Smelting

The smelting process is about extracting the precious metal from its ore. The process yields about 90% of pure gold. However, this percentage of gold is not acceptable in the industry, as it comes with other minerals, such as copper, silver, iron, or aluminum.

Smelting comes as the most crucial step of the entire gold mining. Unless it is done correctly, gold cannot be extracted from its ore. That means there would be no gain. Weight and purity determine the profits from gold.

High pressure is used for gold smelting. Besides, several chemicals and heat are used. All these factors help to crush the gold ore and dissolve the precious metal to segregate it from the impurities. The precious metal needs to be heated to more than 2150 degrees Fahrenheit.

Cyanidation

The Cyanidation process contributes to the recovery of more gold. The process is about oxidation and dissolving of metallic gold in an alkaline cyanide solution. Atmospheric oxygen acts as the oxidant in the process.

The oxidant helps to dissolve the gold and forms sodium hydroxide as well as sodium cyanoaurite. An aqueous solution of sodium cyanide must be present with the oxidant to ensure the same.

Once gold dissolves completely, experts extract the gold-bearing solution from the solids. When it comes to ores with more than 20 grams of gold in every ton of ore, vat leaching is performed to ensure the cyanidation process. On the other hand, heap leaching is done to extract gold from ores of lower gold content.

Since these ores contain gold of extremely low density, vat leaching calls for high amounts of solids and solutions. That is why capital costs can become higher. Hence, experts have found out strategies to avoid the whole separation process of the precious metal.

One of the popular strategies in this regard is to add granular activated carbon to the gold ore slurry in times of or after the finishing of gold solubilization.

Gold Refining

The gold refining process is about extracting as well as separating precious metals in doré from electronics and jewelry (recycled products). Many techniques contribute to pulling out the final impurities so that high caratage gold can be obtained. The key techniques used in this regard include Cupellation, Fizzer Cell, Miller Process, Wohlwill Electrolytic Process, Pyrometallurgical Process, Aqua Regis Process, and Inquartation and Parting.

Experts use the Wohlwill Electrolytic Process only when the initial gold content is around 96%. The Aqua Regis Process only comes up when the initial silver content is below 10%. The Pyrometallurgical Process is used when copper is present in gold.

Gold Refining Techniques

The two most popular gold refining techniques to extract pure gold include the Wohlwill process and the Miller process. The Miller process makes use of gaseous chlorine to pull out impurities during the gold’s melting point. As a result, impurities come into the surface layer of the liquefied, purified gold. In short, The Miller process is simple and fast. However, it yields gold of around 99.5% purity.

On the contrary, the Wohlwill process can produce gold of around 99.99% purity. Experts use electrolysis in this regard. The process is about lowering the casting of impure gold into an electrolyte solution. The solution contains gold chloride and hydrochloric acid. The electric current is essential for the process when it comes to relocating the gold to the cathode. The precious metal is brought back to a highly pure metallic condition in the negatively charged electrode. The impurities remain as a separate residue.

Gold Assaying

The assaying process helps to find out the composition of an item or material. For instance, assaying contributes to determining the composition of coin, jewelry, or bar. Thus, it can calculate the percentage of precious metal contents.

The most dependable assaying technique is fire assay. It can find out the precise content of many precious metals (like gold, silver, etc) in concentrates or ores. It does not work for ruthenium and osmium.

Use of Clay & Fluxes in Gold Processing

A clay crucible is used in the process to melt a gold-bearing specimen. An amalgamation of fluxes, including borax and silica, a reducing agent, and lead oxide (known as litharge) is also used in this regard.

The process makes use of the fluxes to alleviate the oxidic materials’ melting point to enable them to combine. Moreover, the reducing agent alleviates the molten litharge to very delicate drops of lead scattered throughout the charge.

Next, the drops of lead melt the precious metals and then unite them in a mass. The drops eventually start creating a metallic layer at the crucible’s bottom part. After its cooling, a bright metallic bead only remains. The bead comprises precious metals. Then, nitric acid contributes to boiling the bead to melt the silver.

As a result, gold remains. Then, experts take its weight. In the presence of platinum metals, the appearance of the metallic bead show changes. Experts often use an arc spectrograph to determine the concentration of the precious metals in this context.

Karats in Jewelry

“Karat” represents the content of gold in the jewelry field. 24 karats indicate pure gold. That means 1 karat indicates 4.167% of gold content. Hence, 18 karats represent 75% (18 × 4.167) of gold. Fineness refers to parts every thousand of gold present in an alloy. The key techniques used in assaying include Fire Assay, ICP Spectrometry, Density Measurement, Touchstone, and X-Ray Fluorescence.

Use of Fire Assay in Gold Processing

Fire Assay determines only gold with the accuracy of 0.2 parts per thousand. However, some changes are essential when platinum group metals (PGMs) and nickel are present. On the other hand, ICP Spectrometry can find out other elements with the accuracy of 1 part per thousand. There are no limitations though.

Density Measurement can discover only gold. It comes with poor accuracy. Touchstone can find out only gold. The accuracy is 15 parts per thousand. The process is not suitable for hard white gold as well as high carat. X-Ray Fluorescence can also find out other elements with the accuracy of 2-5 parts in every thousand. The process is not more than just flat samples.