With the depletion of mineral resources, the improvement of mechanized coal mining and the change of geological conditions, the coal mined is becoming "lean, fine, miscellaneous and chemical", and more and more fine materials need to be sorted and graded. Flotation can not only recover a large number of low ash clean coal, but also effectively purify circulating water for coal cleaning and improve the effect of other processes. It is recognized as one of the most economical and effective separation technologies and methods for desulfurization and ash reduction of fine coal slime, and it is also one of the best economic, social and environmental benefits for coal preparation plants.

Flotation is a complex physicochemical process, many factors have an impact on the process. The main influencing factors are: slime properties, flotation equipment, flotation reagents, flotation personnel and management system.

I. Slime Properties

1. Coal petrographic composition: According to coal petrographic composition, coal is divided into vitrinite, bright coal, dark coal and silk carbon. The properties and floatability of different coal and rock compositions are different. The contact angle of vitrinite and bright coal is 37▫-40▫ and that of dark coal and silk charcoal is 26▫-33▫ respectively. Therefore, the floatability of vitrinite and bright coal is better than that of dark coal and silk charcoal. Generally speaking, when the content of dim components in raw materials is high, it will increase the difficulty of flotation.

2. Coal metamorphism degree: Coal with medium metamorphism degree has the best floatability. The flotability of young and old coal is poor. One reason is that they have more developed pore, and the porosity is the smallest in moderate metamorphic degree. Second, because of their poor hydrophobicity and low degree of metamorphism, coal with more oxygen-containing functional groups has strong hydrophilicity and poor floatability, while anthracite with higher degree of metamorphism has lower hydrophobicity due to the reduction and shortening of side chains.

3. Coal oxidation degree: Coal has a large porosity and surface area, so its surface has a fairly good adsorption capacity. Oxygen has strong chemical activity. Coal particles can adsorb it in air and water, and make the surface of coal particles oxidize. After coal oxidation, the surface hydrophilicity increases and the floatability decreases. There are two ways of coal oxidation, one is to be oxidized in the natural wind blooming process, the other is to be oxidized in long-term immersion in water, the latter oxidation is more intense, so the residence time of cement in water should be reduced as far as possible, such as direct flotation.

B. Flotation equipment

At present, the flotation equipments widely used in domestic coal preparation plants are mainly mechanical agitation flotation machine and jet flotation machine, flotation column, jet micro-bubble flotation machine, etc. (1) Mechanical stirring flotation machine has obvious effect on coarse slime flotation, but its energy consumption is relatively high, and its selectivity to slime iwith poor flotability, especially fine high ash slime, is poor. It has become the main slime flotation equipment in China. (2). The jet floatation machine can generate 0.05mm micro-bubbles and enhance the process of bubble through the throat to form a "W" shape coal slurry flow form, which can reduce the depth of the water soaking device and maintain the stability of the flotation machine, which is favorable for the two enrichment of bubbles and the foam. Coarse coal particles are deposited in the false bottom. (3) The flotation column has the advantages of simple structure, high efficiency and energy saving, obvious flotation superiority to fine particles and superior separation index, but the flotation effect is poor over 0.3 mm. The components of bubble generator are easy to be damaged and the jet port is easy to be blocked. (4) Jet micro-bubble flotation machine has the characteristics of energy saving, obvious superiority to micro-particle flotation and superior separation index, but the jet opening is easy to block, resulting in insufficient aeration, affecting the flotation effect.

3. Flotation reagents

The problems of large flow rate, low concentration, fine particle size and high consumption are common in slime flotation process of coal preparation plants in China. With the continuous improvement of mechanized mining degree, the amount of the refuse and slime in raw coal will increase continuously. The amount of slime entering into the floating coal will not only increase, but also the flotability of the slime will become worse and worse because of the sliming of the refuse. The requirement of reagent performance for the slime flotation will also be higher and higher.

Current status of flotation reagents:

1. Low flotation yield, high consumption of reagents, low performance-price ratio, and no significant economic benefits of flotation;

2. Pharmaceuticals have strong irritating odor and are not environmentally friendly, affecting the health of employees.

3. Most of them are chemical wastes. Their appearance is black and viscous, with many impurities. They are easy to block the pharmaceutical pipelines and often cause the production responsibility accidents of coal flotation.

The following aspects will be the developing trend of flotation reagents for coal preparation.

1. Easy to use and adjust. Compound flotation reagent is used for single coal, which has both the properties of foaming agent and collector. Only one reagent is adjusted in flotation operation to facilitate operation. For complex and changeable coals, foaming agent and collector are used separately. Our company can flexibly formulate the best medicament use plan according to the coal quality, to ensure the maximum benefit of customers.

2. Energy saving and high efficiency. Most collectors used for flotation in coal preparation plants at home and abroad are petroleum products. Kerosene and light diesel are energy-shortage materials. China's petroleum resources are relatively poor. In order to alleviate the contradiction between energy supply and demand, it is an inevitable trend to improve the performance of flotation agents and reduce their consumption.

3. High performance regulator. Regulators include activators, emulsifiers, catalysts, dispersants, etc. High performance adjuster can greatly improve the flotation effect, not only reduce the dosage of flotation reagent, but also improve the yield of flotation concentrate. In view of different coal quality conditions, it is urgent to develop and study various adjusting agents with different properties.

4. Safety and environmental protection. The product has high flash point, which ensures the safety of workers in transportation, storage and use, effectively reduces the occurrence of production safety accidents in enterprises; while ensuring production, the slurry flotation reagent should be guaranteed.