Gold Refining Plant Model

Introduction to Aqua Regia refining
The aqua regia refining process is used to refine gold alloys using a combination of acids and chemical products. The aqua regia solution consists of a mixture of hydrochloric acid (HCl) and nitric acid (HNO3). The refining process can be broken down into several steps:

• Analysis of the gold to be refined
• Production of Au grains or thin strips needed for the refining process
• Refining process
• Separation and recovery of the AgCl produced in the refining process
• Precipitation of the gold in solution using chemical products
• Filter and rinsing of the gold powder
• Analysis of the refined gold
• Melting of the Au into final required shape (ingots, grains etc)

The Aqua Regia refining method
Best suited for refining gold with an initial purity of at least 600/1000. Initial purities below this value will reduce the production capacity as well as increase process times.
Recommended maximum silver percentage: 10-12 %. Percentages above these values can reduce production capacity as well as increase process times.

The gold refining plant can refine all gold alloys that are within the above stated parameters or thereabouts.

For gold alloys significantly below a certain purity (in our case 600/1000) or with silver content significantly above a certain amount (in our case 10-12 %), the inquartation process is recommended. This refining method uses only nitric acid (HNO3).

For the aqua regia refining method, the gold must be either melted into fine grains – this is normally done using a melting furnace and a grain tank – or very thin strips.
Strips are best because they have a large surface area and dissolve completely when emerged in the aqua regia solution. Grains, due to their irregular shape and size, are more difficult to dissolve and some are often leftover in the reactor at the end of the process. On the other hand, they are very quick to produce.

Waste products of the Aqua Regia refining method
Non-precious, heavy metals left over in the waste solutions can be treated and recovered using a water treatment plant. This unit recovers these metals by producing hydroxides that then have to be sent to specialized companies for treatment and disposal.

Acid fume neutralization
Fume scrubbers use caustic soda or sodium hydroxide (NaOH) to neutralise the acid fumes produced during the refining process.

The solution inside the fume scrubber must be periodically checked. If the pH falls below 10, it needs to be reintegrated with sodium hydroxide. The formation of crystals on the bottom of the tank also means that the solution is saturated with nitrates and must be changed. The solution must be changed and treated according to local environmental/pollution laws.

Chemical products needed for the Aqua Regia refining method
The reagents are:

• Nitric acid HNO3 at 36 Bè
• Hydrochloric HCl at 21 Bè
• Various chemical precipitation products

Results of the Aqua Regia refining method
The aqua regia refining units can guarantee a minimum purity of 999.5. Experienced workers can achieve 999.8. It is important to point out, however, that a purity of 999.9 is only needed for those companies that are either gold bullion dealers or have the “good delivery” certificate. In other cases, 999.5 purity is the market standard.

It is possible to achieve 999.9 by performing a double refining cycle but, as explained above, this isn’t necessary except for particular companies.

Description of the gold refining unit
The refining plant is used to refine gold alloys using the aqua regia method as described
above.

• Refining capacity : 32-40 Kg of grains (see “The Aqua Regia refining method”)
• Purity obtainable: 999,5/1000 ( see “Results of the Aqua Regia refining method”)

The plant is made up of:

• 1 PPl tray where two 100 lt containers for HCl and HNO3 acids are stored.
• Polypropylene hood containing a suction fan to be connected to a fume scrubber.
  Designed to the recover any acid losses.
• 2 100 lt polyethylene containers with cover and suction manifold
• 2 Electrical heaters with potentiometers
• 2 50 lt pyrex reactors with relative 1.5 m2 condensers (useful capacity of reactors : 45 lt)
• 2 Graduated tanks for HCl and HNO3. Acid transfer from the reactors by vacuum
• 1 Filter unit on wheels with a compressed air diaphragm pump to transfer the solutions
• 1 350 lt cementation tank with stirrer, antacid frame and service stairs
• 2 200 lt polyethylene tanks, fitted onto a trolley to store king waters
• 1 Electrical board to control lighting and heaters

Technical data:

• Power supply : three-phase.
• Power : 8 Kw.
• Overall dimensions : 197x130x100 inches (length-width-height)