Froth flotation process

Flotation reagents are used in the froth flotation process to (/) enhance hydrophobicity, (2) control selectivity, (J) enhance recovery and grade, and (4) affect the velocity (kinetics) of the separation process. These chemicals are classified based on utili2ation collector, frother, auxiUary reagent, or based on reagent chemistry polar, nonpolar, and anionic, cationic, nonionic, and amphoteric. The active groups of the reagent molecules are typically carboxylates, xanthates, sulfates or sulfonates, and ammonium salts. 

Xanthates are used in a froth flotation process of soils contaminated with mercury. The soil to be treated is mn through hydrocyclones, and the slurries are flocculated, dewatered, and removed to a secure landfill. The effluent water is recycled. The process is suitable for treating industrial land sites contaminated with mercury droplets (115). 

Flotation Reagents. Three types of chemical reagents are used during the froth flotation process collectors, frothers, and modifiers. 

FIGURE 16.11 In the froth flotation process, a stream of bubbles (white circles) is passed through a mixture of ore (orange circles), rock (brown rectangles), and detergent. The ore is buoyed up by the froth of bubbles and is removed from the top of the chamber. The unwanted gangue is washed away through the bottom of the container. 

Froth-flotation processes are used extensively for the separation of finely divided solids. Separation depends on differences in the surface properties of the materials. The particles are suspended in an aerated liquid (usually water), and air bubbles adhere preferentially to the particles of one component and bring them to the surface. Frothing agents are used so that the separated material is held on the surface as a froth and can be removed. 

Xanthates are used commercially as flotation agents in froth flotation processes for the separation of crushed materials, and their use as vulcanisation accelerators for rubber has already been mentioned (p. 264). 

Zinc and lead usually occur together in nature as sulfides. Earlier separation processes involved the fine grinding of the combined sulfides and then treating the particles with chemical reagents to cause one sulfide to be preferentially wetted and rhns the two sulfides separated by the froth flotation process. In a first stage, the lead sulfide is floated while the zinc sulfide sinks to the bottom of the tank. In the second stage, the process is reversed and the zinc sulfide is floated. Gangue and other nonmetals collect at the bottom of the tank. The separated sulfides are dewatered to a 6-7% moisture content and are referred to as the zinc concentrate and the lead concentrate. 

Froth flotation process for cleaning coal fines in which separation from mineral matter is achieved by attachment of the coal to air bubbles in a water medium, allowing the coal to gather in the froth while the mineral matter sinks (ASTM D-5114). 

Some modifications to the standard froth-flotation process have been developed in order to improve separation and recovery efficiency including carrier flotation, emulsion flotation, and floe flotation. These are all aimed at making very fine particles amenable to flotation, or at improving the efficiency of their flotation. 

Kitchener, J.A. The Froth Flotation Process in The Scientific Basis of Flotation, Ives, K.J. (Ed.), Martinus NijhofF The Hague, 1984, pp. 3-51. 

Until about 1919 the practice of flotation was conducted with much secrecy, and was on a semi-empirical basis. Since then the literature has become voluminous and in recent years the scientific principles underlying it have been fairly well elucidated. A long paper by Sulman (Trans. Inst. Min. and Met., 29,44-204 (1919)) gave the first comprehensive description of the froth-flotation process and showed the fundamental importance of the contact angle. The reader will find excellent text-books in Gaudin s Flotation (1932), Luyken and Bierbrauer s Flotation (1931),... 

Suggested industrial uses of carbohydrate xanthates, apart from the cellulose Viscose process, include the flotation of minerals and the production of plastics. Two patents by Brown and his associate claim an effective purification of both iron ore and silvinite ore by froth-flotation processes employing, for example, sodium starch xanthate, pine oil, and a suitable amine. Silberstein obtained plastic masses from mixtures of sodium dextrin xanthate with urea, formaldehyde, or glyceritol. Starch xanthate has been suggested as a dispersing, wetting, and adhesive ma-... 

In the case of a naturally occurring ore of lead, the first step is usually to concentrate the ore and separate it from other metallic ores. This step often involves the froth flotation process in which the mixture of ores is finely ground and then added to a water mixture that contains one or more other materials, such as hydrocarbons, sodium cyanide, copper sulfate, or pine oil. Air is then pumped through the ore/water/secondary material mixture, producing a frothy mixture containing many small bubbles. 

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gas-phase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow. 

In order for the flotation to be effective, the sludge must be recycled to increase the particle size of the calcium carbonate. This recycle also gives heavier flow in the settling basins, which results in faster settling rates. For the flotation to be effective, the magnesium hydroxide in the sludge must also be completely dissolved by the carbon dioxide and washed out. In the froth flotation process (used to separate the calcium carbonate from the clay, silt, or other water contaminants), an aqueous slurry of the sludge is first conditioned (mixed) with soda ash and sodium silicate to disperse the clay and... 

Many theories have been advanced concerning the mechanisms involved in surfacing the mineral particles so as to create a hydrophobic hydrocarbon film on the mineral surface, and many investigations have been carried out to define these mechanisms. When firoth flotation is used in an aqueous medium that carries the solids to be separated (together with dispersed air bubbles and possibly an organic liquid) a three- or possibly a four-phase system must be considered. In most froth flotation processes, the solid particles are initially completely water-wetted, and the solid-liquid interface must be replaced by... 

Density and contact angle are usually modified in a froth flotation process. Thus, collector oils or surfactants can be added to ores that adsorb on desirable ore particles increasing the contact angle (and promoting attachment to gas bubbles) but that do not adsorb much on undesirable particles (which do not then attach to the gas bubbles). This... 

The abundance of zinc in the Earth s crust averages 70 g ton . Zinc is a chalcophUic element like copper and lead, and a trace constituent in most rocks. Zinc rarely occurs naturally in its metallic state, but many minerals contain zinc as a major component from which the metal may be economically recovered. The mean zinc levels in soils and rocks usually increase in the order sand (10-30 mg kg ), granitic rock (50 mg kg ), clay (95 mg kg ) and basalt (100 mg kg ) (Adriano 1986, Malle 1992). Sphalerite (zinc blende, cubic ZnS) is the most important ore mineral and the principal source for zinc production. Smithsonite (ZnGOj) and hemimorphite (Zri4(Si207) (0H)2 X H2O) were mined extensively before the development of the froth-flotation process (Jolly 1989, Graf 2002). The main impurities in zinc ores are iron (1-14%), cadmium (0.1-0.6%), and lead (0.1-2%), depending on the location of the deposit (ATSDR 1994). 

In industry, froth flotation processes are used to separate particles and/or droplets gas by attaching them to gas bubbles, which rise in a flotation vessel to form a product layer of foam teimed froth. The most common type of froth flotation is induced gas flotation (also termed scavenging flotation), in which gas bubbles are injected (sparged) into the flotation medium. Variations include dissolved gas flotation, in which gas is dissolved in water after which microbubbles come out of solution, attach to the dispersed species of interest and cause them to float (see also Section 8.3 and Chapter 10). 

Some modifications to the standard froth flotation process have been developed in order to improve separation and recovery efficiency including carrier flotation. 

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