Assay Furnaces: How They Work and Are Used for Precious Metal Testing

What is an Assay Furnace?

An assay furnace is a type of furnace used in metallurgy to heat ores and alloying elements to determine the content of precious metals like gold, silver, platinum or palladium. Assay furnaces allow assayers to smelt a small sample of ore, called the assay charge, to extract the precious metals from rock and other impurities. The key characteristics that distinguish assay furnaces from other types of metallurgical furnaces are their ability to reach high temperatures over short periods of time and process small sample sizes with precision.

Types of Assay Furnaces

There are several different types of assay furnaces designed to accommodate different testing needs:

• Muffle furnace - The most common type used in fire assaying. Muffle furnaces use electricity and can reach over 1,600°C.
• Cupellation furnace - Used in cupellation to separate precious metals from base metals and impurities.
• Crucible furnace - Designed to melt metals in clay or ceramic crucibles, allowing separation from slag.
• Fluxing furnace - Uses fluxes to concentrate metals in the slag for analysis.
• Ashing furnace - Ashing furnaces incinerate non-metallic portions of the sample at lower temperatures.

Besides the different furnace types, assay furnaces can use a variety of fuel sources including electricity, coke, oil, propane or natural gas. The specific type of assay furnace used depends on the material being analyzed, desired temperature range, testing capacity needs and the fuel source available.

Key Components

While styles vary, most assay furnaces have common components that enable them to heat to high temperatures quickly and maintain a consistent heat for testing procedures. These include:

• Refractory lining - Protective inner lining made of insulating refractory bricks or ceramic fiber blankets.
• Heating elements - Metal heating coils or electrodes that heat up when electricity flows through them.
• Burner - Gas-fuelled component which produces a flame within the furnace chamber in fuel-fired assay furnaces.
• Chimney - Exhaust flue to remove smoke and fumes produced during the assay process.
• Door - Heavy weighted plug door, lift door or guillotine door to insert and remove the crucible.
• Crucible support - Ceramic or metal stand to hold the crucible and contents inside the furnace.
• Thermocouple - Temperature sensor used to monitor and control furnace temperature.

How Assay Furnaces Work

In fire assaying, a sample of crushed ore is added to fluxing chemicals and a reducing agent in a ceramic crucible or cupel. This mixture, called an assay charge, is designed to separate the precious metals when heated to high temperatures.

The assay charge is placed in the furnace chamber on the crucible support and the furnace door is sealed shut. The furnace is then preheated to a starting temperature around 250°C to dry out the assay charge. Once dried, the temperature is increased over a period of an hour or more up to between 1,000-1,200°C.

At these high temperatures inside the assay furnace, the reducing agent causes base metals like lead and copper to oxidize, forming a molten slag. The flux helps bind to gangue minerals from the ore sample. This leaves the dense precious metals concentrated at the bottom of the crucible.

After the smelting process, the crucible is removed from the assay furnace and poured into a mold to form lead buttons containing the precious metals. These lead buttons then undergo cupellation or additional chemical steps to separate and quantify the gold, silver or other metals extracted from the ore.

Uses and Advantages

The main uses of assay furnaces include:

• Determining the grade of newly discovered ore bodies at mines.
• Testing mineral extraction processes and efficiency.
• Evaluating the purity of refined precious metals.
• Verifying the precious metal content in jewelry, cutlery, coins or other metal wares.
• Separating precious metals for use in coins, bullion, jewelry and electronics.

Key advantages of using assay furnaces for these applications include:

• Small sample size - Only a small amount of ore is needed for furnace testing.
• Speed - Short heating times and rapid temperature changes allow for fast assaying.
• Accuracy - Carefully controlled temperatures ensure accurate separation and collection of precious metals.
• Repeated testing - The furnace can run many assay charges in a day for high-volume analysis.
• Adjustable temperature - Furnace temperature can be set based on the material being assayed.
• Minimal slag and fumes - A properly configured assay furnace produces less waste compared to other smelting furnace designs.

Maintenance and Safety

To keep assay furnaces operating safely and efficiently, regular maintenance is essential:

• Inspect refractory - Check for damage and repair worn areas with refractory cement.
• Clean exhaust system - Remove built-up soot and residues from chimney flues.
• Replace heating elements - Over time, heating coils or electrodes wear out and must be replaced.
• Calibrate temperature controls - Make sure thermal sensors and controllers are precisely measuring furnace temperature.
• Check fuel supply - Confirm adequate flow and pressure for gas-fueled furnaces.
• Test door seal - Doors must seal tightly to achieve maximum temperatures.

Proper safety gear and precautions are also critical when working with high temperature assay furnaces:

• Fireproof clothing - Wear a smelter jacket, leggings, heavy gloves and a face shield.
• Respirator - Use an approved respirator to prevent inhalation of dust and fumes.
• Ventilation - Operate furnace under a fume hood or system to remove exhaust gases.
• Secure work area - Clear clutter and combustible materials away from the furnace.
• Careful loading/unloading - Use sturdy, long handled tongs when inserting or removing crucibles.
• Scheduled breaks - Take regular breaks when running the furnace continuously to avoid fatigue.

Conclusion

Assay furnaces allow geologists, miners, refiners and other metallurgists to accurately determine the composition and purity of precious metal ore samples and refined products. When used properly, they provide a safe, efficient method for assaying small amounts of materials through smelting at high temperatures. With routine maintenance and observance of safety protocols, assay furnaces can deliver reliable, consistent results over many years of service.

FAQs

What temperature do assay furnaces reach?

Assay furnaces can reach temperatures between 1,000-1,200°C, with some specialized models capable of exceeding 1,600°C. The high temperatures ensure complete smelting of the sample and separation of precious metals.

How long does an assay take?

A typical precious metal ore assay takes 1-2 hours from start to finish. The furnace is preheated, followed by a slow ramp up to the operating temperature over 30-60 minutes. Once at temperature, the smelting itself takes 15-30 minutes before sample is poured and cooled.

What size samples can be processed?

One of the advantages of assay furnaces is their ability to analyze very small sample sizes, ranging from just 10-100 grams. This enables testing of drill cuttings or chips from initial exploration without the need to extract large bulk samples.

What safety equipment is required for assay furnaces?

Proper safety equipment includes fireproof smelting jackets and leggings, heavy duty gloves, respirators, and face shields. Proper ventilation and secured, clutter-free work areas are also critical when operating assay furnaces.

How often should assay furnaces be maintained?

To keep assay furnaces working properly, basic maintenance like inspecting refractories, cleaning flues, replacing worn parts, and testing controls should be performed at least monthly. More thorough annual maintenance is recommended as well.