Okrushko Kseniya

Faculty:   Institute of Mining and Geology

Departament:     Mineral processing

Speciality:   Mineral processing

Scientific Supervisor:
  Doctor of Technical Sciences, professor Pavel V. Sergeev

Theme of master's work:

Research of Technology of selective flocculation of coal using synthetic latexes

Introduction

Relevance of the topic

The growth of the level of mechanization of coal production and its further treatment leads to the growth of small group part and mineral contaminants in composites produced. Nowadays the coal slacks treatment is one of the main coal enriching problems which is caused by use of traditional technologies incapable to promote necessary rates of substance segregation. The matter is that preparation of fine fraction is characterized with low selectivity and this is the exactly factor that influence the range of coal preparation efficiency. This fact is the main one that define the growing efficiency of fine and small fraction coal preparation in the whole.

Today the main part of slimes is to be treated through the foam flotation[4]. But this process has low efficiency in case of high fine fraction with size less then 50 mkm content in the initial intake. It leads towards contamination of concentrate as well as waste. The alternative process of coal slime flocculation-flotation preparation presupposes a prior treatment of intake flotation with latex flocculants. This process is more efficient and less wasteful comparing to existing technologies of slime treatment. Nevertheless nowadays all the activities in the field of the coal selective flocculation with synthetic latex need to be improved and fuller theoretical explanation of some stages of the process. That is why a further elaboration of the selective fine coal flocculation process with synthetic latex theory and the process technique improvement on this basement are necessary. Therefore they become an urgent theoretical and practical issue.

Objective

Aim is to improve quality of coal fine-grained material through the introduction in the technological scheme of benefication technology of selective flocculation. The key point here is the use in the process of selective flocculation of synthetic latex, which today are among the most expensive and productive flocculants. The paper will also be disclosed and proved both theoretically and practically the benefits of synthetic latexes as flocculants.

The Promising Process of Fine-Fraction Coal Preparation

Selective flocculation is a process set of minerals fine fraction aggregation to plagettes of 100-300 mkm size with the help of flocculant reagents of different nature. The last ones are usually input into water dispersion of fine feed stock which undergo intensive agitation in turbulent operating conditions of hydromixture stirring.

In suspension turbulent flow selective flocculant fixation on the surface of fractions of definite nature is followed with their further joining with the help of flocculant into plump spatial structures known as plagettes. The range of affinity between flocculant and fraction surfaces has a marginal importance. The separation of flocculated product from dispersed fractions can be performed with the help of gravity, floatation or magnetic methods.

Selective flocculation is a general-purpose process and can be used for different types of minerals: ferrous metal ores (particularly for manganese ore), nonferrous materials oxides and sulfides, rare metals slimes, auriferous gravel, nonmetallic mineral slimes. Depending on a sort of reagent used there are four main types of selective coal flocculation: with the help of inorganic electrolytes, oils, water-soluble and hydrophobic polymeric compound. A type of reagent influences the process results dramatically for it defines aggregative development mechanism.

While selective flocculation of fine-dispersed coal fraction with the help of water-soluble and hydrophobic polymeric compound is used bridged mechanism of structure formation is taking place. Among water-soluble flocculants there are anionic, cationic and nonionic polymeric flocculants. Process of water-soluble polymer and coal surface mutual reaction is based on initiation of long-range ion-electrostatic combinations of flocculants ionogenic groups with the similar structures on the surface. Specific hydrogen bonds and powers of chemical nature are not so important and play the second role (for they are appropriate only on condition of flocculants fixation in adsorbing layer of surface electric coating).

The most promising selective flocculants among other polymeric compounds are synthetic latexes (Picture 1 Electronic photomicrography of butadiene-styrene latex).

Pic.1 — Electronic photomicrography of butadiene-styrene latex [3].

They are presented with aqueous dispersions of rubber latex colloidal particles (50-200 nanometers, picture 1), which are stabilized with the help of different emulsifying agents (most frequently with the help of ionogenic surface-active agents of anionic type)[1].

Latexes are the most typical representatives of lyophobic colloid systems, which are characterized with dispersed phase of high hydrophobic properties. These properties are caused by nonpolar character of structural monomeric units. Rubber-latex globules appear due to the process of emulsion polymerization and are sized in the range of 0,08-0,3 micrometer, they have high aggregative stability, and they are almost water-insoluble. Aggregative stability of latex globules of ionogenic emulsifying agent protective coat imparts negative charge to the globules surface. Colloid-chemical properties of latexes determine efficiency of flocculation-flotation process. Colloid type of the polymer causes high range surface activity. Its hydrophobic property premises efficiency of latex systems selective interaction with naturally hydrophobic coal phase.

According to the theory of colloid system stability (the DLVO theory named after Derjaguin and Landau, Verwey and Overbeek) based on attractive and repulsive forces interaction, coagulation techniques are divided into slow and fast coagulation. Ion-electrostatic interactions produce repulsion, and intermolecular dispersing interactions produce attraction. Depending on correlation of these forces two techniques of latex globule coagulations can appear. Fast coagulation mechanism is typical for particles which have lost considerable parts of emulsifier protective coat, and potential barrier is very low. This coagulation is irreversible and can not be used for selective flocculation process. Meanwhile Partial desorption and higher potential barrier enable particles interaction at a longer distance. This type of coagulation is known as the so called “slow” coagulation, and it is acceptable for selective flocculation process for it leads to formation of chain branching which promote particles adhesion. In this case hydrophobic areas of globules surface are the most active ones. Emulsifying agent washing out into a water phase, its desorption leads to a constant interaction of hydrophobic surfaces with chain structures[5].

According to the data [2] desorption process accelerates rapidly due to selective adsorption of emulsifying agent free ions from suspension to mineral particles surfaces. This provides conditions for latex aggregates fixation and their interaction with coal particles.

To define on what condition this mechanism can be implemented we analyze mathematical equation of the DLVO theory applying it to particles of spherical shape[5]:

,

where Ec is a total energy of two spherical latex globules interaction in a liquid environment (water); εа is an absolute dielectric constant of water; d is a latex globule diameter; φδ is potential of a double electric coat diffusive component on a surface of coal particles; χ — reversed Debye radius; h — distance between globules.

According to the graphical analysis of the equation we should note that:

— Both latex globules size growth as that of their repulsion energy barrier occur simultaneously. It causes aggregative stability increase.

— Polymeric chain formation between coal particles is possible when latex globule surface potential is equal orhigher than 50 microvolt. If it is less than that latex globules loose their flocculation capability because of aggregative stability rapid disappearance.

Latex selective flocculation is a difficultе and multistage process which is determined with a range of technological factors. The factor model depicts process main input and output flows.

On the ground of a priori information which was received from technical literature and some preliminary study it was established that the most important input parameters on usual condition of butadiene-styrene latex usage in concentration plants are those of flocculant consumption, coal-water suspension density, coal ash content and flocculation duration.

Huge researches in the field of coal selective flocculation and synthetic latexes were dine by Ukrainian coal-chemical institute in Kharkov in the nineties. With the help of Voronezh synthetic rubber plant and Voronezh branch of All-Russian research institute of synthetic rubber the scientist of Ukrainian coal-chemical institute in Kharkov developed and tested 17 latex reagent flocculant. The most efficient of them were scientifically and industrially approved in a range of coal factories of Donets Basin.

Lately intensive research work on selective flocculation with synthetic latexes have been done in Donetsk national technical university. Some range of laboratory experiments performed with modern physicochemical research methods helped to understand the working mechanism of latexes as coal flocculants. And the experiments give an opportunity for the further development of the theoretical basis of the process.

In cooperation with Ukrainian coal-chemical institute in Kharkov the approbation of latex flocculants were performed in the coal-cleaning plants of Ministry of Coal Industry of Ukraine. The results of theoretical and experimental researches is shown in the source of information [1].

Among foreign researches in the field we can name further ones:

— Reserches in the field of combined flocculation-flotation technology of coal preparation with ash and sulfuric agents[8],[7].

— latex usage as modifier (waterproofing agent) of oxidized coal while it flotation preparation. The modifier act as water emulsion of sodium polyacrylate with molecular weight of 105-106. It is used in combination with water-oil emulsifier and foaming reagent of alcoholic type. Latex emulsion provides coal particles of the size +0.1 mm withdrawal to float fines while preparation of slimes of the size 0.2-0.5 mm [9].

— Researches on polymeric dispersion reagent (phase type SD-C) usage in flocculation of fine category coal. The peculiarity of this technology is mutual usage of dispersion reagent SD-C and selective flocculant of organic weight (Floc-67 etc)[10].

Summary

We should assume that selective flocculation with hydrophobic reagents (such as oils and synthetic latexes or their substitutes) is the most promising technology of all existing ways to improve efficiency of fine coal slimes treatment.

Literature

1.  И.М. Нікітін, П.В. Сергеєв, В.С. Білецький. Селективна флокуляція вугільних шламів латексами./ — Донецк: ДонНТУ, «Східний видавничій дім», 2001. — 152с.

2.  П.В. Сергєєв, В.С. Білецький. Селективна флокуляція вугілля гідрофобними реагентами /[Електронний ресурс]. — Режим доступу: http://ruthenia.info/txt/biletskv/kn/3.html

3. Р.Э. Нейман. Очерки коллоидной химии синтетических латексов.// — Воронеж: Воронежский государственный университет,1980.— 236c.

4. В.І. Залевський. Селективна флокуляція вугільних шламів синтетичними латексами.[Електронний ресурс] — Режим доступу: http://www.nbuv.gov.ua/ard/2001/01zvissl.zip

5. К.Н. Окрушко, П.В. Сергеев. Исследование устойчивости латексных систем в процессе флокуляционно-флотационного обогащения угольных шламов.// Научные работы Донецкого национального технического университета. — Донецк, ДонНТУ, 2010.

6.  Additive compositions for recovering coal fines by froth flotation : Заявка 2225260 Великобритании, МКИ4 B 03 D 1/00 / Brookes G.F., Spenser L.; Forspur Ltd.— № 8924014 ; Заявл. 14.05.87 ; Опубл. 30.05.90.

7.  Recovering coal fines: Заявка 2190310 Великобритании, МКИ4 В 03 В1/04, В 03D 1/02 / Brookes G. F., Spencer L. ; Fospur Ltd . — № 8711401; Заявл. 14.03.87 ; Опубл. 18.11.87.

8.  Elzeky M., Bavarian F., Attia Y.A. Feasibility of selective flocculation / froth flotation process for simultaneous deashing and desulfurization of high-sulfur coals // Proc. and Util. High-Sulfur Coals : 3-rd Int. Conf., Ames, Lowa , Nov.14-16, 1989.— Amsterdam etc., 1990. — P. 209-219.

9.  Treatment and recovery of larger particles of fine oxidized coal. Патент США , кл. 209-116 ( B 03 D1/02) № 4222861, Заявл. 08.06.78 № 913974, опубл. 16.09.80.

10.  Mengxiong G., Changyin G. The study of mechanisms of selective dispersion flocculation in fine coal processing // Biotechnol. and Separ. Syst. Proc. Int. Sump., Sun Francisco, Calif, 1986.— Amsterdam, 1987.— P.511-524.