In Vietnam, most of the exploited coal is anthracite. About 27 million tons of ROM(Vietnam run-of-mine) coal were exploited in the year 2005. To reach required quality of holders, the ROM coal is cleaned at coal preparation plants by screening, blending, and washing. At present, there are 5 coal preparation plants in Vietnam with 12 million tons of production capacity. In the year 2005, they produced 3.6 million tons of fine coal with low quality (22% ash content) . As per the design, the quality of the fine coal is improved by spiral cyclone systems. However, the spiral cyclone system operated at low efficiency due to problems with density of feed solution. Therefore, the plants have now stopped operation of the spiral cyclone system. At present, the coal preparation plants produce fine coal without any methods of quality improvement.
Quality Improvement of Fine Coal in Vietnam by Flotation Method
Thus, the ROM coal in Vietnam includes high rate of fine coal, especially fine coal with size 0.1 mm . Rate of fine coal in 2005 doubled in comparison with the amount of fine coal in the year 2000 . The coal preparation plants now produce high amount of fine coal with low quality. Therefore, it is necessary to find a way to address this problem.
Flotation method has been used to recover fine coal (0.6 mm) for over 50 years over the world . In broad summary, the carbonaceous mineral constituents of coal can be made to preferentially attach to fine bubbles and float to the surface of dilute slurry, where they can be removed, whereas the low carbonaceous inert minerals of the raw coal do not attach to the bubbles .
This paper examines the washing ability of froth flotation method applied to the fine coal product of Vietnam. In this research,
fine coal samples were taken from a major coal preparation plant in Vietnam.
Profile of Coal Preparation Plant
The biggest coal preparation plant in Vietnam, with total production capacity of 6.1 million tons in the year 2005, was selected as the study site. The cleaning technology of the plant includes 3 jig machines, density medium (DM) cyclone machines, classified cyclones, spiral cyclone system, thickener cyclones, dewatering cyclones and compressed belt. According to the design, the fine coal product of the jig machines is transferred to spiral cyclone system to improve quality. In Vietnam, all coal is separated by DM cyclones with solution having 1.52 g/cm3 of relative density. However, as the spiral cyclone systems operate at high efficiency if the relative density of coal is 1.73 g/cm3, they operate at low efficiency because the operating relative density is different from the design one. Therefore, the plant stopped operation of the spiral cyclone system since 1995. The fine coal is produced directly without any improvement methods. In the year 2005, the plant produced 1.8 million tons of fine coal having 22% of ash content. However, the fine coal product did not reach quality requirement of demand holders. Therefore, the plant faces some problem of fine coal product.
Methodology
To examine washing ability of the flotation method for the fine coal product of the plant, experiments of fine coal washing by flotation method were conducted. As per the results of experiments, the fine coal should be stirred with petroleum for 3 minutes, with froth creator for 1 minute and the froth should be removed for 3 minutes. Therefore, the experiment was implemented to determine collecting agent consumption, froth creator consumption, rate solid/liquid of the fine coal solution, and time for froth flotation process.
a) Objective of the experiment: The fine coal product of jig machines of the study site was taken for the experiment. The fine coal product of the jig machines had high ash content and high amount of fine coal (<0.01 mm). As stated earlier, it is difficult to improve quality of the fine coal product by DM cyclone or spiral cyclone.
c) Collecting agent: petroleum. The characteristics of petroleum are:
- Density: 0.8 g/cm3;
- Relative viscosity: 1.09;
- Rate of hydrocarbons: 15%;
- Rate of hydrocarbon naphthene: 27%;
- Rate of hydrocarbon paraffin: 58%
d) Froth creator: Turpentine oil 1%
At present, all coal preparation plants of Vietnam face problem of fine coal. Each year there are 3 million tons of fine coal produced directly without improved methodology in Vietnam. The fine coal product has high ash content (22 – 30 %) which results in high amount of coal slurry. Moreover, the problem of fine coal can cause other problems for the coal preparation plants, especially environmental problems such as high volume of lost water, coal slurry treatment, etc. It is expected that the flotation method can help the coal preparation plants to solve these problems.
At present, Vietnam has 5 coal preparation plants. The fine coal product from the plants could be cleaned up by flotation method.However, the flotation method is still new in the coal preparation industry of Vietnam. Therefore, it is necessary to put quality improvement of the fine coal by flotation method in practice. This paper estimates the ability and benefits from application of flotation method in the biggest coal preparation plant in Vietnam. This plant has 3 jig machines to clean ROM coal, each operating separately. Therefore, it is suggested that the coal preparation plant should apply flotation method to one jig machine for examining the washing ability at full-scale before applying to the other two plants.
As per the inventory, fine coal product of one jig machine is 91.17 tons per hour. It is expected that the flotation method can wash 597,345.84 tons per year.
