Sampling

This point is the first step of any mining project. When sampling an ore body for the purposes of metallurgical tests and mineralogical studies, the samples must be selected on the basis of how they will affect the treatment process or any metallurgical operation to be included in the design. In this way, rock break properties and gold mineral liberation are key aspects of the comminution circuit and they should be correlated with gold grade. It is well know that deposits have different ore types and its variation in terms of hardness and metallic content can be huge. A proper mineralogical evaluation of the samples selected to characterize the ore body is useful so that the geologist and metallurgist can consider the different ores within the mineralized deposit. Some variations or changes in mineralogy are due to the presence of intrusives or any geological occurrence. Then, an evaluation of the material is an important part because each ore type and mineralized zone represents a significant part of the material that will be sent to the plant.

Examination of mineral can help to determine the identity, character, grain size and associations of the valuable minerals with non-valuable minerals. The information will indicate liberation size analyses for the comminution and process recovery selection. The analysis can be based on the microscopy of polished sections or stereoscopy for mineral sizes. If some minerals are difficult to be studied, specific and special equipment can be employed. Sometimes, minerals are in complex associations and this factor is important to consider due gold recovery can be affected.

Detailed information on the gold distribution in the deposits is important. Also, the distribution of other metals and non-metallic minerals are important aspects to be considered. Obviously, the person in charge of the recovery process considers this information a key point of the process. The person in charge of the comminution circuits may assess the first problems of the comminution circuit and the need for additional circuit design parameters. For example, the presence of clays or sulphate affects the comminution and recovery process, the fist produces slimes and the second one acidity.

With preliminary information will be assessed the limits and reserves of the deposits. Later, when more information is available, the initial assessment will be actualized. The mining engineer creates a mine model based on geological parameters such as ore types or lithology which have to be related to the gold distribution and minor metals present in the ore body. Process modeling, plant expansion or production is developed with this information. There is no doubt that metal values are variable and the sampling procedure is a key aspect.

The relatively cost of obtaining samples, performing the tests and the time employed are crucial factors in the selection of the process and forecast the gold production variability through life of mine. In the early stages of the project, samples are analyzed for gold and other elements. Some people are used to employ the rejected material in order to save money and time. However, this is a bad practice due to the sample was prepared for chemical assays and not for metallurgical test. Although, samples must be crushed, the procedure is different. Basically the production of fine material is different and this aspect has an effect on the metallurgical results. Next table shows cyanidation results obtained with the same sample, but prepared under different objectives. Cyanide consumption is affected by the presence of fine material and all samples from chemical assay preparation produced more material minus than 400 mesh. Also, the cost is affected. See next table.