F F Aplan and B J Arnold WET FINE PARTICLE CONCENTRATION

Coal Preparation 5th Edition. Editor Joseph W. Leonard, III. Published by socienty for mining, metallurgy and exploration, inc. Littleton, Colorado 1991.

PART 2: WET FINE PARTICLE CONCENTRATION SECTION 3: FLOTATION

by F. F. Aplan and B. J. Arnold

INTRODUCTION

Coal flotation is a fine cleaning process usually present in all but the older preparation plants. It is typically used to clean eitliei the ā28 mesh (ā500 mkm) or ā100 mesh (ā150 mkm) raw coal fines, and it is the only commonly used preparation method that can effectively recover coal particles finer thau say, ~200 mesh (74 mkm). To place coal flotation into its proper context, it should be noted that the bulk of the raw coal sent to preparation plants is treated by gravity concentration. On a worldwide basis, from 0 to 40% of the raw feed to a preparation plant is treated by flotation, while in the US only about 10% of the raw coal feed is floated. Roughly, 50% of US preparation plants use flotation to process at least a portion of the raw coal feed, although most of the newer plants incorporate flotation into their flowsheet.
Historically, the process has only been used to treat the raw coal fmes naturally occurring after mining and a minimal amount of crushing at the preparation plant. This is in sharp contrast to ore flotation where, typically, the entire tonnage is first ground and then floated. As a consequence, of the approximately 500 Mt. of raw coal treated by preparation in the US yearly, it, is estimated that only about 40 Mt are sent to the flotation circuits (producing ā 30 Mt of clean product). This small tonnage compares with about 400 Mt of ores treated annually in the US by flotation. By analogy to ore flotation, where it is an effective, large tonnage, low cost, easy to operate process, its potential in coal preparation should be great. Reasons why it has not achieved a greater potential in coal preparation have been attributed by Aplan to:
1.Indifferent results in removing sulfur and ash.
2.Difficulty in floating low rank and oxidized coals.
3.Inability to make clean separations with the finer sizes.
4.Inability to clean slurries with a high percentage of clay,
5.Problems with water chemistry.
6.Competition with effective, gravity coarse-cleaning methods.
7.Low value of coal historically.
8.Dewatering problems.
9.Transportation and storage problems with coal fines.
10.Marketing problems with coal fines.
11.Inadequate attention to the process in the plant.
12.Inefficient use of capital and operating monies.
13.Inadequate attention to procedures, machines, and processes.
14.Lack of an adequate R&D program
15.Inertia.
A detailed discussion of these problems has been given, and it is suggested that they may be solved by an aggressive research, development, plant testing, and plant process control program. The greater need for low sulfur coal may be expected to increase the raw coal treated by flotation since the liberation of more of the pyritic sulfur and ash-forming minerals will require additional grinding of the raw coal.

The Variability of Coal

Luckie has said that "There is no such thing as coal, there are coals," since coal shows a high degree of variability depending upon source and genesis. This is reflected in the great variation in the physical and chemical properties of coal as a function of rank. The change in the physical and chemical properties of coal with ASTM coal rank, based on the work of Given. It is apparent that coal rank is directly Ifelated to such factors as % C, % O, % H, vitrinite reflectance, calorific value, etc., and, over a shorter range of ranks, by many other properties as well. As a consequence of these compositional variations, the surface and flotation properties of coal may be expected to vary widely from source to source. Fortunately this variation with rank is largely predictable, unless other factors, such as coal oxidation, have intervened. How the nature of raw coal influences its cleaning may be found in the literature. The great variability in coal makes it extremely difficult to extrapolate data from ore flotation practice and, often, from one type of coal to another.

MINERALOGY, CHEMISTRY, AND SURFACE CHEMISTRY OF COAL FLOTATION

Reagents ę frothers

The purpose of coal flotation reagents is to create stable bubbles, to alter the natural or induced hydrophobicity of coal, pyrite, or ash forming minerals, or to alter the flotation operating conditions, through, e.g., dispersion, flocculation, acidity control, etc. Not infrequently, a reagent may influence more than one of these functions, and its effect may differ markedly depending on the concentration used.
The primary function of a frother is to produce a copious supply of bubbles of the proper size and at modest cost. The bubbles produced should have sufficient toughness and persistence to provide the transport of coal particles from the slurry to the froth, but they should not be so tough and persistent as to prevent their destruction by water sprays in the froth launder or to interfere with the subsequent pumpability of the froth concentrate. The frother should have no collecting power for any particle other than the particle to be floated. Since one of the major goals of coal flotation is to achieve selectivity between coal and gangue particles, there are many subtle properties of the frother which can either enhance or restrict its ability to selectively float only the desired particles. The frother should also be readily dispersible in water. The most commonly used frothers for coal flotation have been cresol, pine oil, alcohols (e.g., MIBC), and the polypropylene glycol ethers (e.g., Dowfroth 250).
Historically, the first coal flotation frother used was probably an impure cresol, a byproduct of coke making. There are several problems with the use of this frother:
1) it absorbs strongly onto coal, so reducing the frother concentration in solution as to require relatively large amounts of this reagent;
2) commercial grades used for frothing are relatively impure and have a substantial, but highly variable, collecting power for coal (ostensibly due to its variable benzoid compound content);
3) it may pose environmental and health problems. For these reasons it is rarely used as a frother today.
Pine oils, whose principal constituent is terpineol, have also been popular frothers for coal. They have suffered from the defect of composition variability due to the source of the conifer used for their production. They also are known to have a significant, but variable, collecting power.