Surfactant coal slurries

For flotation to be most effective, reagents such as oil (collector) and surfactant (frother) must be added to the aqueous coal slurry. The collector adsorbs on the coal surface and makes it more hydrophobic, while the surfactant facilitates production of a stable froth. According to Leonard (26), the optimum coal sizes for froth flotation are between 50 and 140 mesh (0.3 and 0.106 mm). While conventional froth flotation will successfully remove minerals from coal, it has limited effect on the pyrite content, as pyrite tends to float almost as well as coal. Because of this, researchers have been looking for better ways to remove pyrite from coal. In addition, as previously mentioned, it is necessary to grind coal to very fine sizes in order to sufficiently liberate the mineral matter and pyritic sulfur. But, at these very fine sizes,... 

Coal slurry is metastable colloidal system. Its stability is determined by coal mineral components, mainly finely divided hydrophilic clays and surfactant additives. The expenditures for surfactants and homogenization constitute a significant share of the cost of fuel. Furthermore, in the course of the combustion of WCF, evaporation of each 10% of the water in its composition requires to bum 1% of the present coal. Aside from a relatively low calorific value, WCF tends to freeze during transportation in winter conditions. 

Surfactants are also becoming more important in the coal mining industry. Aside from flotation processes, they are also employed as binders for the suppression of coal dust, and as dispersal aids and antifreezes for coal slurries that are pumped through pipelines. 

Use of Surfa.cta.nts, Although the use of steam to improve dewatering is consistently beneficial, the effects of surfactants on residual moisture are highly inconsistent. Additions of anionic, nonionic, or sometimes cationic surfactants of a few hundredths weight percent of the slurry, 0.02—0.5 kg/1 of soHds (50), are as effective as viscosity reduction in removing water from a number of filter cakes, including froth-floated coal, metal sulfide concentrates, and fine iron ores (Table 2). A few studies have used both steam and a surfactant on coal and iron ore and found that the effects are additive, giving twice the moisture reduction of either treatment alone (44—46,49). 

Table 2 Types of surfactants that have been used for preparing highly concentrated coal-water slurries Many of these surfactants are used in combination with formulations that depend on the characteristics of the coal...

Fig. 7 Simplified schematic of the Beijing Coal Water Fuel Plant, China. Clean coal is first ground in multiple stages to produce a graded size distribution that will fluidize with the minimum amount of water. Drum filters are used to remove excess water from the slurry, and surfactant reagents are combined with the slurry using pug mills. This plant processes 35 metric tons of coal per hour and produces a slurry that is 65% coal by weight. (From Ref.. )...

Methods for burning mixtures of pulverized coal in oil (variously called colloidal fuel, coal-in-oil slurry, or coal-oil suspension) have been studied for nearly a century and require the production of coal-in-oil suspensions. Stable short-term suspensions of coal in residual fuel oil are easily attained if the coal is pulverized to 200 mesh (75 am) and, by adding surfactants, long-term stability can be obtained so that the coal will not settle out of the mixture even over periods as long as a few months. The interaction between the coal and the hydrocarbon allows the apparent viscosity of the coal-oil mixture (COM) to be 10 times greater than the fuel oil base and special precautions may be taken to provide adequate pump capacity, heating systems for the slurry, and properly sized burner nozzles. 

Morway (1965) obtained a patent for using a hydrocarbon oil with a small percentage of an imidazoline surfactant to coat coal particles uniformly. After adding this mixture, the coated coal can be mixed with water. The water weight concentration can be reduced to 20%. This slurry with low overall moisture is easier to heat at the final discharge point prior to combustion than plain coal-water mixtures. 

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