Main reagents and their functions in fire assay
Release time:
2021-01-19
Fire Assay Main reagents used and their functions
Fire Assay Various reagents must be added to separate the precious metals to be determined from the matrix components in the sample through high-temperature melting. The functions of the various reagents added are different. Some can capture precious metals in the sample after chemical reaction at high temperature, which are called collectors. Some can melt the sample and combine with the matrix components to form silicate, borate and other slags, which are called fluxes or melting agents/slagging agents. According to the role of reagents in the smelting process, fire assay reagents are divided into seven categories: flux, reducing agent, oxidizing agent, desulfurizer, sulfurizing agent, collector and covering agent. Some reagents have only one use, such as SiO2 which is only used as an acidic flux, but other reagents have several different uses, such as PbO which is both an alkaline flux, a collector and a desulfurizer.
Flux
The function of the flux is to melt the refractory Al2O3, CaO or silicate matrix components in the sample and generate a good slag, thereby decomposing the sample. According to the chemical properties, fluxes are divided into three types: acidic, alkaline and neutral.
1. Silicon dioxide, or quartz powder, is a very strong acidic flux.
2. Glass powder is a commonly used acidic flux, which can be used to replace silicon dioxide powder. In addition to the acidic component SiO2, glass powder also contains alkaline components such as CaO and Na2O. Therefore, its acidity is weaker than that of quartz powder. Generally, 2-3g of glass powder is equivalent to 1g of SiO2. It is usually made from plate glass, washed and dried, and then crushed to 0.246mm-0.175mm in a grinder.
3. Borax is a lively and easily fusible acidic flux. It begins to lose its crystalline water at 350℃ during melting and expands rapidly. Therefore, excessive use of borax in the batching is easy to cause material overflow during melting, resulting in the loss of samples in the crucible. Borax can form borates with many metal oxides, and their melting points are lower than those of the corresponding silicates. For example, the melting point of CaSiO2 is 1540℃, the melting point of Ca2SiO4 is 2130℃, and the melting point of CaO·B2O3 is only 1154℃. Adding borax to the batch can effectively reduce the melting point of the slag.
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