Tsevma Nataliya

Contents

1) Introduction.
2) Methods of intensifying the process of dewatering of fine coal.
3) Dispersion and stability of mineral suspensions.
4) A theoretical analysis of the mechanism of action of dehydrating agents, water repellents and flocculants.
4.1) Surface-active substances.
4.2) The mechanism of interaction of surfactants with carbon particles.
5) Trends and problems of research.
5.1) Theoretical studies.
5.2) Experimental studies.
6) References.

Introduction

In today's concentrators important part of the process is dehydration.

Dewatering operations of small dressing products can be divided into three groups:condensation and dehydration under the influence of gravitational forces, mechanical dewatering and thermal drying.

Methods for the intensification of these processes can be varied.
With increasing degree of mechanization of mining operations increased amount ofdetail, which is extracted and enriched. Of particular importance intensification ofdehydration has a small dressing products for mining and mining and chemical industries, due to the fact that the grinding is subjected to a deep almost all the raw materials that goes to the processing methods of magnetic separation and flotation, followed by dehydration. In practice, enrichment is used as a flotation for the enrichment of sludge, and for the timely removal of fine silt from the circulating water, which createsconditions for the closure of the water-sludge cycle. The number of small dressing products are subject to dehydration.

Significant prospects are physico-chemical methods of intensification of the existing equipment that will improve the efficiency of dehydration without significant capital investment. Experience of industrial chemicals, the introduction of intensifiers in themineral processing plants gave positive results. Thus, the use of reagent-intensifiers is one of the most important factors in addressing the improvement of water and sludgemanagement at the concentrators and the prevention of environmental pollution. As agents, intensifiers can be used inorganic electrolytes, low molecular weightsurfactants and synthetic petroleum, water-soluble polymers, and viscous organic liquids.

Methods of intensifying the process of fine coal dewatering

Methods of intensification of the processes of dehydration of small dressing products may be different. In this paper we consider only the physical-chemical methods of intensifying the processes of mechanical dewatering and filtration process mainly small dressing products. These techniques allow not only to increase the performance of filtration products, but also reduce the removal of solids from the filtrate and in some cases to reduce the moisture content of sediment. Reduction of flotation concentrate removal increases the yield of marketable products, and leads to improved technological performance enrichment processes in general by reducing the degree of zashlamlenosti recycled water.

In recent years, attracted the attention of specialists of the heat-physical methods of intensifying dehydration, which are in use during the dry sediment was deposited on the grid, the hot gas (products of burning fossil fuels, atmospheric air). Although the use of hot gas has the same goal as the heating of the suspension before filtration, it is not only more efficient but also economical. The advantage of this method is due? Primarily low-cost heat for heating a relatively small amount of water covering the surface of the inner cavities of sediment after the exclusion of these fluids. Most of the moisture due to its short duration of contact with the hot gas is heated slightly. However, this heating is sufficient to change the viscosity of water and the weakening of its energy to the surface of the material, resulting in a hydrodynamic flow of water under the pressure of the gas, which moves in the capillaries.

Sludge dewatering with superheated steam was carried out under industrial conditions.For this method is characterized by minimal loss of heat from exhaust air, high security and technological efficiency. In the superheated steam contains no oxygen, so theoxidation or combustion of coal is possible. Drying with superheated steam gives the effect of dehydration of the porous sediment.

Img 1 — Dependence of the moisture content of sediment on the temperature of superheated steam

Intensify the process of dewatering of fine coal filter apparatus can also change the structure of filter cake, or a decrease in the binding energy of the particle surface with water. Under the restructuring meant an increase in sediment porosity and a decrease in its specific surface area due to the aggregation of fine particles under the influence of coagulant (electrolyte) and flocculants (organic compounds and water-soluble polymers).Adding hydrophobic reagents according to the type of oils provides an increase in contact angle q, and the use of surfactants lowers the surface tension of water.

As a result, the height of capillary rise of water and the capillary pressure decreases, and therefore the number of small coal-held moisture decreases. The use of surface-active substances, together with a decrease in surface tension of water increases the wettability of the particles, and hence to increase the height of capillary rise of water in the sediment layer, filtered, and an increase in humidity. Thus, for better dewatering reagents must be used, which reduces the surface tension of water is reduced and the wettability of coal.

Coagulation of fine coal, which is preceded by its filtration, leads to the fact that the smallest particles, incorporating a relatively large floccules, increases the porosity of thesediment. Thanks to increasing its permeability and accelerates the formation of a layeron the filter. Surface-active substances under certain conditions contribute to the emergence of osmotic pressure, which increases the rate of fluid filtration and capillaryphenomena of osmosis, reduces the moisture content of sediment during drying. Based on the foregoing, we choose to intensify the process of fine coal dewatering of surface-active substances (chemicals, water repellents), and flocculants [2].

Dispersibility and stability of mineral suspensions

Slurry concentration and hydrometallurgical processes are polydisperse systems of particles with a size from one-hundredth of a micron to several millimeters. Depending on the size of the particles, which dominate, distinguish coarse-grained, fine-grained, silty shlamisti and pulp. In coarse-grained include pulp, which is dominated by particles larger than 0.5 — 0.3 mm. Fine-grained particles are smaller than the pulp with 0.5 — 0.3 mm.Referred to as sludge suspensions with particles of size less than 50 — 70 microns, and sometimes produce the most delicate part of the sludge — sludge with a particle size less than 10 — 5 microns. In the coal industry is often referred to the sludge particles less than 2 mm.

To include slurry pulp, which take precedence over other sludge particles quantitatively or qualitatively. At the same characteristics attributed to the clay slurry with appropriate quantitative or qualitative benefits of sludge. The boundary between the suspensions and colloids in the range size of 0.1 — 1.0 microns.

Thus, to include the actual suspensions dispersed systems with a predominance of particles larger than 5 microns. They are not colloidal dispersions, however, have much in common with the properties and much more practical value than the ash. When the acid leaching of gold ores and concentrates, non-ferrous, rare, radioactive and other metals are enriched suspension colloidal dispersions of silicon and aluminum acid. In alkaline slurry hydrates appear oxides of iron, aluminum and other metals. Stabilization of dispersed fractions of ore slurries by a variety of mechanical effects in the processes of treatment (intensive mixing, grinding), especially at elevated temperatures. Chemical peptization occurs when injected into the pulp chemicals that contribute to hydrophilization of the particles and increase their surface electric charges (water glass, the field of phosphate), reduced the concentration of the coagulating electrolyte by washing the precipitate with water, changing the other properties of the medium, which leads to an increase in the charge of colloidal particles and their dispersion .

The stability of dispersed systems is one of the central issues of colloid chemistry.Consideration of this issue of suspensions is associated with the intensification of the processes of condensation and filtration [1].

Theoretical analysis of the mechanism of action of dehydrating agents, water repellentsand flocculants

Surface-active substances

Surface-active substances are called chemical compounds, such that when dissolved or dispersed in a liquid selectively adsorbed at the interface, in turn, defines a set of physico-chemical or chemical properties that are of practical importance.

The main physico-chemical properties of surfactants, which are based on many processes, including those that run on concentrators, include: reduction of surface tension, surface activity, the critical micelle concentration, wetting and hydrophobization, emulsification, foaming, stabilization and others.

The properties of surfactant affects not only the number but also the order of connection of the individual atoms that make up molecules.

Properties, which are different surfactants are:

— Reduction of surface tension in a very dilute solution due to adsorption and orientation of the molecules at the interface;

— The formation of micelles in the destruction of the corresponding concentration of the solution

SAC by reducing the free energy of the system.

img 2 — micelles [5].

All synthetic surfactant is oil and water sensitive compounds, consisting of hydrophobicand hydrophilic parts.

Are part of the hydrophilic carboxyl-SOO, OSO-3 sulfate and sulfonate SO-3 groups, as well as a combination of hydrophilic residues with ether groups — CH2 — CH2 — O — CH 2 -CH 2 -, polyglycolic — (SN2SN2O) n, or groups that contain nitrogen.

The hydrophobic part consists mainly of paraffin chain, straight or branched, benzene ornaphthalene ring with alkyl radicals.

It can be argued that the use of surfactants is almost universal, because every production process in one way or another associated with the interaction of the surface.

Synthetic surfactants are used at present are divided into four classes:

— Anionic surfactants — compounds that dissociate in aqueous solution to form anions,causing the surface activity. Among them are the most important linear alkyl benzene sulfonates, sulfates, and sulfoefiry fatty acids;

img 3 — anionic surfactants [5].

— Amphoteric (ampholytic) surfactants — compounds that are ionized in aqueous solutionsand lead, depending on conditions (mainly on the pH — medium), ie in acid solutionexhibit the properties of cationic surfactants in alkaline solution — anionic surfactants.Among the principal amphoteric surfactants should be noted alkylbetaines,alkilaminokarbonovye acid derivatives of alkyl imidazolines, alkilaminoalkansulfonaty;

img 4 — amphoteric surfactants [5].

— Non-ionic surfactants — compounds that dissolve in water, without ionizing. The solubility of nonionic surfactants in water is explained by the presence of functional groups. As a rule, they form the nitrates in aqueous solution as a result of hydrogen bonds betweenwater molecules and oxygen atoms of the molecule polietilenglikolovy surfactants. These include polyglycol esters of fatty alcohols and acids, polyglycol esters of fatty acid amides, alkyl polyglycol ethers alkylamides.

img 5 — Non-ionic surfactants [5].

— Cationic surfactants — compounds that dissociate in aqueous solution to form cations,determining the surface activity. Among the cationic surfactants are the most importantquaternary ammonium compounds, imidazaliny, fatty amines.