The metallurgical performance of the recovery process is influence by the gold ore preparation and the importance of this process is many times overlooked for metallurgists, operators and analysts. The process start considering the environmental factors, which are in connection with the location of the gold processing plant and the recovery process selected. In this way, air and water pollution problems, accentuated by proximity to population centers and streams, affect decision on plant location and processing methods. Regulations in several countries need mill owners to assume responsibility for continued maintenance of waste disposal areas even after the plant has been shut down and dismantled, thus affecting the preparation of the material and the tailings disposal system.
Gold grade selection and blending during mining operations and in the preparation of stockpiles assist in smoothing the processing plant. In this way, marginal ore is often stockpiled separately and may be mixed with high gold grade ore or under the proper circumstances may be treated by an especial recovery process. Basically, concentration of a gold ore by physical methods may offer several advantages resulting from the decreased weight of material to be handled and from the elimination of undesirable constitutes in the mill feed. Physical concentration methods either employed include grinding and sizing, gravity, magnetic, flotation and some sorting techniques. All gold ores don’t respond well to any of these processes and must be handled the material according to the process. Gold concentration to any useful extent cannot be performed without losses to the tailings. Many gold operations are based on the division or classification of materials according to the extraction process. For example, low and high slimes content or high and low sulphides fractions.
It has been noted that chemical upgrading at the plant site involves concentration and/or leaching the ore and generate tailings with low gold content. Upgrading by this consideration is important in order to evaluate the cost of each process involved in the treatment. Sections with high costs must be optimized. In this way, each activity or action performed during the initial steps of the recovery process has an impact. The basic steps are gold ore weighing, moisture determination, crushing, sampling for gold content, blending and grinding. Some gold ores may be partially dried, if otherwise too wet to handle. Most gold mills weigh and obtain moisture samples as the ore is received. This information could be obtained after the ore is crushed and enroute to the milling operation.
Crushing is performed in conventional equipment (e.g. jaw crusher, cone crusher, gyratory crusher) to reduce the ore to the range between ¾” and 4”, depending of the plant size and type or process. In most plants the crushed ore is stored temporarily in bins to help in blending and regulating the feed to the milling operation that follow. If the material is floated and/or leached, the grinding operation is performed by SAG, Ball or Rod mills in open and closed circuits. Grinding size is determined by gold ore properties and process requirements and may vary from 60 to 80% passing 75 microns.