Froth flotation depressants

Mining. Numerous patents have advocated the use of alkanolamines in mining appHcations. Triethanolarnine has been used as a depressent in the flotation of copper (164), in the electrotwinning of gold (165), and as an aid in the froth flotation of nickel ores. Phosphate ore flotation has been improved through the use of a fatty acid condensate with ethanolamine (166). Beneficiation of tin ore has been accompHshed using fatty acid alkanolamides (167). 

A special kind of flotation is froth flotation (124). Selective depressants for the separation of ABS and HIPS have been used. Acetic acid, methanol and quebracho have been probed as selective wetting agents for froth flotation. All these compounds are potentially selective agents, provided the particles are of similar size. The use of quebracho at pH 11 gave the most promising results as selectivity was seen over a wide particle size range. 

Other types of water effluent problems can arise from froth flotation operations. For example, with multimineral flotation when separate concentrates are required for copper, lead, and zinc, copper would normally be floated first, then lead, and finally zinc sulfide. This separation sequence requires addition of sodium cyanide as a depressant. The waste water stream from this process can contain 50 mg/L or more of dissolved cyanide which has to be detoxified before discharge. Calcium hypochlorite has been found to be effective to neutralize the cyanide by oxidation to cyanate (Eq. 13.29). 

Depressant Any agent that may be used in froth flotation to selectively reduce the effectiveness of collectors for certain mineral components. See also Froth Flotation. 

Flotation reagents are selected to produce a stable froth and adjust the affinity of target minerals to collect in the froth. Other reagents depress the collection of minerals in the froth. The froth containing the copper minerals overflows into collection launders. 

The pH of the pulp to the flotation cells is carefliUy controlled by the addition of lime, which optimizes the action of all reagents and is used to depress pyrite. A frother, such as pine oil or a long-chain alcohol, is added to produce the froth, an important part of the flotation process. The ore minerals, coated with an oily collected layer, are hydrophobic and collect on the air bubbles the desired minerals float while the gangue sinks. Typical collectors are xanthates, dithiophosphates, or xanthate derivatives, whereas typical depressants are calcium or sodium cyanide [143-33-9] NaCN, andlime. 

The flotation process is applied on a large scale in the concentration of a wide variety of the ores of copper, lead, zinc, cobalt, nickel, tin, molybdenum, antimony, etc., which can be in the form of oxides, silicates, sulfides, or carbonates. It is also used to concentrate the so-called non-metallic minerals that are required in the chemical industry, such as CaF2, BaS04, sulfur, Ca3(P03)2, coal, etc. Flotation relies upon the selective conversion of water-wetted (hydrophilic) solids to non-wetted (hydrophobic) ones. This enables the latter to be separated if they are allowed to contact air bubbles in a flotation froth. If the surface of the solids to be floated does not possess the requisite hydrophobic characteristic, it must be made to acquire the required hydrophobicity by the interaction with, and adsorption of, specific chemical compounds known as collectors. In separations from complex mineral mixtures, additions of various modifying agents may be required, such as depressants, which help to keep selected minerals hydrophilic, or activators, which are used to reinforce the action of the collector. Each of these functions will be discussed in relation to the coordination chemistry involved in the interactions between the mineral surface and the chemical compound. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

At least two types of flotation have been attempted on spodumene pegmatites. Conventional flotation, in which the valuable mineral is discharged in the froth and the worthless gangue is depressed, utilizes cationic soaps as the frother (20). More recently a rather unconventional type of flotation, in which most of the worthless components are floated off in the froth, was developed (1), and appears to be more successful. 

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