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

[0019] Another important objective of the invention is the provision of improving the rate at which water is removed so that given dewatering equipment can process higher tonnages of particulate materials.

[0020] An additional objective of the present invention is the provision of novel fine particle dewatering methods that can reduce the moisture to a level that no thermal drying is necessary.

[0021] Still another object of the instant invention is the provision of a novel dewatering method that creates no adverse effects on up-and downstream processes when the water removed from the dewatering processes disclosed in the present invention is recycled. SUMMARY OF THE INVENTION

[0022] The instant invention discloses methods of rendering the particulate materials suspended in water hydrophobic and / or enhancing the hydrophobicity of the materials, so that the process of removing the water by mechanical processes such as filtration and centrifugation are improved. The improvements will result in lower product moisture and / or higher throughput. The essence of the invention is to render the particles reasonably hydrophobic in the first place by suitable means and, then, add non-ionic low HLB surfactants to significantly enhance the hydrophobicity of the particulate materials, so that the pressures required to expel the moisture from smaller capillaries are reduced substantially. This will greatly increase the rate of dewatering and reduce the cake moisture.

[0023] Much of the hydrophobicity enhancing reagents disclosed in the present invention have HLB numbers below approximately 15, and are insoluble in water. Therefore, appropriate solvents such as light hydrocarbon oils and short-chain alcohols may be used in conjunction with the low-HLB surfactants. The light hydrocarbon oils, which should also be considered as HLB surfactants, may also act as hydrophobicity enhancing agents. Furthermore, the packages of the reagents used in the instant invention are capable of lowering surface tension. Also, the particles coagulate owing to the increased hydrophobicity and, thereby, increase the capillary radius. Thus, the reagent compositions disclosed in the present invention is capable of increasing contact angle, lowering surface tension, and enlarging capillary radius, all of which should contribute to decreasing capillary pressure and improving dewatering. The instant invention also discloses reagent dosage by adding cations, and achieving substantial moisture reduction by spraying reagents to filter cake and applying mechanical vibration during 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.