Winning the Metals
Digging ore from the earth is only half the battle. The processing of the ore is often just as challenging and expensive, which takes place in refineries, smelters and mills. quartz powder plant
While the inside of a mill seems to be a very baffling maze of tanks, pipes, pumps, conveyors, motors, chemicals, pulps and solutions to the visitor, this seeming confusion is actually a carefully designed system constructed for only one objective and purpose - to recover the valuable minerals which are locked up in the ore. mineral beneficiation plant
The end product from a mill is called a concentrate, or a doré bar of the metal itself in the case of gold and silver.
All milling and concentrating processes begin with a grinding and crushing stage, which most of the time represents most of the total cost of processing the ore.
Ore minerals are generally found within and among grains of other minerals of ore or relatively gangue minerals which are worthless. This fact makes the processing of some ores more complicated than others.
For example, to the metallurgical engineer, a complex sulphide ore containing microscopic particles of sphalerite within small blebs of galena or other sulphides presents a special challenge, in other words, to design a milling process that will liberate these different constituents from each other as economically and cleanly as possible so that each may be recovered.
Nitric acid is regenerated and recycled to the process. Chemical oxidation with acid nitric works at atmospherical pressure, 90 oC, and residence time of 1 to 2 hours into the reactor.
The process makes a high oxidation of iron, sulphur, arsenic, antimony, and any type of sulphide material present. marble grinding machine
The main advantages are: to work under atmospherical pressure, to employ air instead oxygen, good recoveries of silver, and to promote formation of iron arsenate into the solution.
Also, there are disadvantages such as reuse calcium nitrate, formation of elemental sulphur and its interference during cyanidation. With the years have been implemented a step called Hot-Lime which destroys or dissolves sulphur. This step must be carried out before cyanidation. Finally, it is necessary a good recovery of condensation water and a careful water balance in order to get optimum results.
The type of grinding mill is identified by the type of grinding media utilized. Each type of mill has characteristics that adapt it to certain grinding applications. The operational characteristics of each mill type must be evaluated in relation to the objectives of the grinding requirements being studied in order to select the optimum mill. The common mill types are discussed in the following sections.
The type of grinding mill is identified by the type of grinding media utilized. Each type of mill has characteristics that adapt it to certain grinding applications. The operational characteristics of each mill type must be evaluated in relation to the objectives of the grinding requirements being studied in order to select the optimum mill. The common mill types are discussed in the following sections.
Rod mills can accept feed up to approximately 2” diameter and generally are selected to grind product to the – 4 mesh to – 35 mesh range. Grinding actions is by line contact between rods extending the length of the mill. Rods tumble and spin in roughly parallel alignment and in so doing simulate the crushing and grinding action from a series of roll crushers. Large particles spread the ends of rods and is so doing impart an additional action termed scissoring. These actions result in preferential grinding of coarse material and a minimum production of slimes and excessively large material termed tramp oversize. A relatively uniform sized product is produced. Due to the nature of the grinding action, rod mills can sometimes advantageously replace the fine crushing of damp or sticky material that tends to coat the faces of crushing surfaces. Rod mills can produce a final product on sand brick, lime, or coke breeze where the feed contains low moisture and is often ground dry. In the metal mining industry most rod mill applications involve wet grinding were material is reduced in size from crusher product size to a size suitable for ball millfeed. Rod milling in the size range utilized is more efficient than ball milling in that the desired product is obtained at a lower cost per ton.
