In metallurgy and chemical analysis, an assay is a process used to determine proportions of precious metals in ores and other metallurgy substances such as silver or gold. It is important to use a precise method when identifying pollutants present within a sample, especially when certain metals in low concentrations indicate a high level of pollution in a mining site.
What is an assay used for?
An assay test determines the impurities present before sending a substance to a sorting facility. In conjunction with improved chemical-assay techniques, we use ‘robolabs’ at our concentrators and smelters.
These robotic laboratories provide rapid feedback to plant operators so that the dosage of reagents and other important parameters can be adjusted to maintain the optimum economic recovery conditions for the treated materials.
The assaying process is important to potential investors as the results can affect stock prices. Assayed metals are required to meet strict purity standards which often correspond to existing investment contracts. When assay results indicate the potential value of a mineral or ore extraction site, results may trigger a stock increase and value reassessment for the company. Conversely, poor mineral assay results can trigger a decline in stock prices.
How does the assay process work?
There are more than 200 analytical techniques that determine precious metal concentrations. The 3 most common techniques are:
- Fire assay
- Wet chemistry
- Instrumental Analysis
Fire assay is the classical technique used to determine the precious metal content of silver and or gold ore and secondary concentrates. It is more time-consuming and elaborate than instrumental analysis, but it is generally more accurate.
Fire Assay Methods
Fire assay has been used by engineers for thousands of years. Pyrometallurgical methods today employ high-temperature electric lab furnaces to process gold and silver ore samples, secondary scrap metals, and most platinum group metals. It uses 3 steps:
Step 1: Preparation
The fire assay process begins by grinding samples into a fine powder to mix them with dry powder chemicals. Initial samples are sometimes evaluated by XRF to determine the rough composition, which helps the assayer choose which reactive chemicals to add and how much. The sample is then weighed before dry-reactive chemicals are mixed in.
Step 2: Collection (Crucible Fusion)
The mixture is heated in a ceramic or metal container called a crucible, which heats the contained samples at extreme temperatures until all of the powders fuse into a glass-like slag. Then, complex reactions occur to collect all of the precious metals in an alloy.
Activity and viscosity are the important properties of the waste matter created by the fusion process. Once the reaction is complete, the contents are poured into a mold and allowed to cool.
Platinum group metals are much more difficult to analyse using fire assay. Other metals or metal compounds, such as copper-nickel sulfide, are sometimes added as collectors when platinum group metals are present.
Step 3. Separation (Cupellation)
The mixture is then separated in a fire assay cupel, typically a shallow cup made of phosphorous, bone ash, or calcium phosphate. Once cupellation is complete, the cupel will reveal a small bead of precious metals. The bead is then weighed and analysed further for precious metal content using an instrumental analysis technique, such as ICP-AES.
Wet Chemistry Assay Methods
In a wet chemistry assay, the ore sample is dissolved in a solution in order to analyse it. Different chemicals are used depending on the combination of elements present. This is also the method used to selectively separate and isolate alloys and elements.
Wet chemistry includes classical analytical techniques like dissolution, titration, and gravimetric analysis. Although wet chemistry methods require more steps, they’re more accurate and precise than instrumental analysis for higher concentration materials, second only to fire assay. Here are some details about gravimetric analysis, the most common wet chemistry assay method:
Gravimetric analysis with precipitation is the most widely used wet chemistry method. After precipitation, the material is filtered, washed, dried, and weighed to determine its mass. The mass of the precipitant is then compared to the determined mass to identify the percentage of metal present.
