Mining in focus
Leon Louw
In copper mining, about 80 % of the produced quicklime is used in flotation processes.
is also dependent on lime products as binder and neutraliser of acidity. Lime products are used in the petroleum industry as thickener, sealant, and pH regulator. Thermal power stations use it for sulphur sorbent or detainer, depending on the process.
Addition of lime as binding agent is used in silicic and lightweight bricks. It is also the reacting agent in lightweight concrete, and plasticiser and displacer in mortar. Furthermore, lime products are used in asphalt paving to assist with waterproofing, stabilising, and as anti-disintegrant. In the agricultural sector, lime-related products are used in soil improvements as a pH regulator and accelerant for plant nutrients. In fertilisers, it serves both as deodoriser and nutrient. It is added to insecticide and fungicide as a diluent.
Lime products have a wide application in smelters, where it is used to act as flux and binder. It also has environmental applications in the absorption of smoke gases and chimney gases and is used during bar unmoulding, as nickel precipitation catalyst, and wire production lubricant. More applications include the cyanidation of auriferous( gold) and argentiferous( silver) minerals, while it also prevents the generation of hydrocyanic acid in the cyanide hydrolysis and is used as alkalising agent in cyanidation. During alkaline flotation, it is used as pH regulator, pyrite depressant, calcium salts precipitant, and linkage of the active silica contained in clays, among others. It is also used in leaching gravels to
neutralise the residual acidity and it stabilises the clays contained in the ore.
As part of non-metallic mining activities, lime products are used as pH regulator in salt and nitrate products and as linking agent in boric acid production. It is used as the binder in obtaining granulated ulexite( fertiliser) and potassium nitrate and as a linking agent and depressant of the magnesium present in the brines containing lithium.
Quicklime production
Quicklime is produced by the conversion of limestone through calcining in rotary or vertical kilns. During the lime process, for every tonne of saleable quicklime produced, about two tonnes of‘ pure’ limestone or six tonnes of impure limestone is consumed. The limestone consumption is dependent on the type of product, limestone purity, degree of calcing, water temperature, and the quantity of waste products. For every part of lime produced, two parts carbon dioxide( CO₂) are generated. A distinction is made between‘ brown lime’ and‘ white lime’, having available lime contents of 68 % and 72 %, respectively.
Soft-burned lime is the most reactive and is difficult to produce because of the delicate operating balance that must be achieved, sufficient heat to drive off the CO₂ without overheating closing the pore structures. Depending on the kiln age and design, between 200kg( new kilns) and 350kg( old kilns) of coal is required to produce one tonne of quicklime.
Application in copper production
The limestone used for metallurgical purposes must be of superior grade, have a silica and alumina content of less than 2 %, and it should have a low sulphur and phosphorus content. For fluxing, the iron content may be high, but the material must be lumpy and finely crystallised to avoid decreptitation. In a powdered form, the milled carbonates can be mixed with ore and pressed as self-fluxing pellets.
Quicklime as a flux removes impurities such as phosphorus, silica, and sulphur. In the beneficiation of non-ferrous metals, lime is used to control pH in the flotation process, to neutralise iron sulphides, and to extract and recover metals through precipitation or leaching processes.
Quicklime and hydrated lime are widely used in the flotation or recovery of many non-ferrous ores, in particular copper ore flotation in which lime acts as depressant and maintains proper alkalinity in the flotation circuit. Lime is also used in the flotation of zinc, nickel, and lead-bearing ores. In the smelting and refining of copper, zinc, lead, and other non-ferrous ores, noxious gas fumes of SO₂ can be neutralised by passing these gases through‘ milk of lime’( dilute hydrated lime in an aqueous suspension) in a scrubber to avert the formation of sulphuric acid in the atmosphere and corrosion of plant equipment.
Quicklime is used to neutralise the acid effluents generated by the acid plants associated with copper
[ 28 ] MINING MIRROR JUNE 2018