The time required to refine gold ranges from a few hours for a single batch to several days, depending on the desired purity level and the refining method employed. Gold refining removes impurities like silver, copper, and other base metals from crude gold to achieve a high fineness, typically 99.99% or higher. Understanding the duration requires considering the entire process, from receiving the raw material to producing the final, certified bar. The overall timeline is influenced by both the chemical reaction time and preparatory and logistical steps.
Pre-Refining Preparation and Smelting
The refining process requires proper preparation of the raw material, a stage that typically consumes several hours to a full day. Raw gold arrives in various forms, such as recycled jewelry, electronic scrap, or semi-pure doré bars from mining operations, which are 80% to 95% pure. This material must first be melted down in a furnace, known as smelting, to create a homogeneous liquid metal mixture.
Smelting involves heating the material to gold’s melting point of 1,064°C (1,947°F) or higher. Fluxes like borax and soda ash are often added to help separate base metals and impurities into a glassy slag layer. Once the molten metal cools, a sample is taken for assaying, a chemical analysis that determines the exact composition and starting purity. This analysis is necessary before selecting the most efficient refining path.
Accurate assaying is a time investment that speeds up the overall process by identifying which contaminants need to be targeted. The results dictate whether the gold proceeds directly to a high-purity process or requires rapid initial purification. While physical melting and casting take a few hours, the laboratory analysis for a precise assay can significantly extend the preparatory phase before the main refining reactions begin.
Industrial Refining Methods and Cycle Duration
The time spent in the core refining stage depends on the specific industrial method chosen, balancing speed against final purity. The two dominant large-scale methods are the Miller Process and the Wohlwill Process, each having a distinct cycle duration. The Miller Process is a chemical method that involves blowing chlorine gas through the molten gold.
This chlorination causes base metals and silver to react, forming a slag of metal chlorides on the surface of the molten gold. The Miller process is fast, often completing a batch in three to six hours. This rapid chemical action typically produces gold with a purity of about 99.5% to 99.6%, which is not the highest standard.
For gold requiring ultra-high purity, such as 99.99% or 99.999%, the slower Wohlwill Process is necessary. This electrochemical method uses impure gold cast into an anode and immersed in an electrolyte solution of chloroauric acid. An electric current is applied, which slowly dissolves the gold at the anode and deposits extremely pure gold onto a cathode. This reaction generally requires 24 to 48 hours, or longer, to complete a refining cycle.
Factors Influencing Total Processing Time
The total time to refine gold extends beyond the chemical reaction time due to several logistical and material factors. The scale of the operation is a major variable, as large industrial refineries process enormous batches requiring more time for handling, heating, and cooling. For instance, the sheer volume of metal and the time needed to heat a large furnace to over 1,000°C and then cool the product adds hours to the timeline.
Material Complexity and Purity
The complexity of the starting material also affects the timeline. Gold with high levels of contaminants, such as silver or other platinum group metals, requires additional steps. If the initial doré bar has low gold content, it may need multiple purification cycles or preliminary chemical treatment before the main refining process.
The specified final purity acts as a time multiplier. Achieving a fineness of 99.99% requires the extended duration of the Wohlwill process, while 99.5% can be achieved quickly with the Miller process. Even after the chemical reaction is complete, logistics like re-melting the refined product into marketable bars and obtaining final certification add to the total elapsed time.