Methods of enhancing fine particle dewatering

Abstract

A new method of improving the process of dewatering fine particulate materials is disclosed. In this method, an aqueous slurry of fine particles is treated with appropriate hydrophobizing reagents so that the particulate material becomes moderately hydrophobic with its water contact angle considerably below 90°. A low hydrophile-lipophile balance (HLB) number surfactant is then added to the slurry, so that the surfactant molecules adsorb on the moderately hydrophobic surface primarily by hydrophobic attraction and, thereby, increase its contact angle close to or above 90°. By virtue of the greatly enhanced hydrophobicity, the water molecules adhering to the surface are destabilized and removed more readily by a mechanical dewatering process. Any nonionic surfactant with its HLB number below about 15 may be used for the hydrophobicity enhancement. The surfactants may be used in conjunction with appropriate solvents such as light hydrocarbon oils and short-chain alcohols. The moisture reduction can be further improved by using appropriate electrolytes in conjunction with the low HLB surfactants, spraying surface tension lowering reagents onto the filter cake, subjecting the cake to a suitable vibratory means, and by using combinations thereof.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 09 / 327,266, filed on Jun. 7, 1999, the entire disclosure of which is incorporated herein by reference.BACKGROUND [0002] In the mining industry, run-of-the-mine (ROM) ores are crushed and pulverized to detach (or liberate) the valuable components from waste rocks. Although ROM coal is rarely crushed, a significant portion is present as fine coal. The pulverized ores and fine coal are then separated using appropriate methods. One of the most widely used methods of separation is froth flotation. In this method, a pulverized ore (or fine coal) is mixed with water to form a slurry, to which surfactants known as collectors are added to render selected constituent(s) hydrophobic. For the case of processing higher-rank coals such as bituminous and anthracite coals, which are naturally hydrophobic as mined, no collectors may be necessary. When these materials are not sufficie...

AI Technical Summary

Benefits of technology

"The present invention provides methods for decreasing moisture in fine particulate materials during mechanical dewatering processes such as vacuum and pressure filtration and centrifugation. The invention improves the rate at which water is removed, allowing higher tonnages of particulate materials to be processed. The invention also reduces the moisture level to the point where thermal drying is not necessary. The invention does not create any adverse effects on up-and-downstream processes when the water removed from the dewatering process is recycled. The invention involves rendering the particulate materials hydrophobic and then adding low-HLB surfactants to enhance their hydrophobicity, reducing the pressure required to expel moisture from smaller capillaries and increasing the rate of dewatering. The invention also includes using light hydrocarbon oils and short-chain alcohols as hydrophobicity enhancing agents and spraying reagents to filter cake and applying mechanical vibration during the drying cycle time."

Problems solved by technology

However, this process is inefficient, particularly when the mineral (or coal) particles are fine.

Very often, these products need to be further dewatered in a third and the most costly step, i. e., thermal drying, which may be an option for high-priced materials.

However, it is not so for low-price commodities such as coal.

This represents a loss of valuable natural resources and causes significant losses of profit to coal producers.

In this case, coal producers can no longer afford to discard the fines.

It is unfortunate that there are no technologies available today, other than the costly thermal drying, to lower the moisture of coal fines.

If a filter cake contains capillaries of different radii, it would be more difficult to remove the water from the finer capillaries, At a given pressure drop applied across the filter cake, one can see that the water trapped in the capillaries that are smaller than certain critical radius (rc) cannot not be removed.

Recognizing that dewatering is essentially a de-wetting process, it is difficult to see how one type of reagents can be used for both.

Thus, high HLB surfactants can lower the surface tension, but they can also dampen the hydrophobicity and decrease the contact angle.

For this reason, the high HLB surfactants used as dewatering aids can actually cause an increase in moisture content.

Furthermore, the reagents remaining in filtrate eventually return to the flotation circuit and cause adverse effects.

However, most of the flocculants used as dewatering aids are hydrophilic.

Therefore, the method of using polymeric flocculants for dewatering has limitations.

The use of flocculant and ionic surfactants may be beneficial in dewatering, but they could dampen the hydrophobicity of the particles and, hence, adversely affect the process.