Drying and gas or vapor contact with solids – Process – Diverse types of drying operations
Reexamination Certificate
1999-06-07
2003-03-04
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Diverse types of drying operations
C034S334000, C034S338000, C034S340000, C034S349000, C034S380000, C034S397000, C210S770000, C210S772000, C210S771000, C530S344000, C530S345000, C530S410000, C530S419000
Reexamination Certificate
active
06526675
ABSTRACT:
BACKGROUND
Many particulate materials are produced and processed in aqueous media. Before they are sold to customers or further processed, it is often necessary to remove the water. Dewatering can be achieved by either mechanical methods (e.g., filtration and centrifugation) or thermal drying. In general, the former is cheaper than the latter. However, mechanical dewatering becomes inefficient with finer particles. Dewatered products contain high moistures, often requiring thermal drying to meet specifications.
In a given mechanical dewatering process, bulk of the water is removed rather quickly. What is difficult to remove is the water adhering to the surface of the particulate material. Thus, the amount of the residual water left in the product is approximately proportional to its surface area. For a given material, specific surface area is inversely proportional to the square of its particle size. Therefore, the residual moistures in filtered products increase accordingly with decreasing particle size. A more quantitative explanation for the difficulty in dewatering fine particles by filtration may be given by the Laplace equation:
Δ
p
=
2
γ
cos
θ
r
,
[
1
]
in which &Dgr;p is the pressure of the water inside a capillary (formed between the particles present in a filter cake), r is the capillary radius, &ggr; is the surface tension of water, and &thgr; is the contact angle of the particles in the cake. The contact angle is a measure of the hydrophobicity (water-hating property) of the particles. Eq. [1] shows that the pressure required to blow the water out of a capillary increases with decreasing capillary radius. Considering that finer particles form smaller capillaries, one can see the difficulty in dewatering fine particles. With a given filter cake, which consists of particles of different sizes, there must be a distribution of capillaries of various radii. At a given pressure drop applied across a filter cake, it would be difficult to blow the water out of the capillaries whose radii are below certain critical value. Thus, the number of capillaries, whose radii are below the critical radius, should determine the final cake moisture.
Various polymeric flocculants are used to enlarge the particle size and, hence, minimize the number of smaller capillaries. Electrolytic coagulants can also be used to enlarge particles. Groppo and Parekh (Coal Preparation, 1996, vol. 17, pp. 103-116) showed that fine coal dewatering improves considerably in the presence of divalent and trivalent cations. They found this to be the case when using cationic, anionic and nonionic surfactants.
Eq. [1] suggests also that capillary pressure should decrease with decreasing surface tension and increasing contact angle. Various surfactants are used to decrease the surface tension. Most of the dewatering aids used for this purpose is ionic surfactants with high hydrophile-lipophile balance (HLB) numbers. Sodium laurylsulfate and sodium dioctylsulfosuccinate, whose HLB numbers are 40 and 35.3, respectively, are typical examples. Singh (Filtration and Separation, March, 1977, pp. 159-163) suggested that the former is an ideal dewatering aid for coal because it does not adsorb on the surface, which in turn allows for the reagents to be fully utilized in lowering surface tension. The U.S. Pat. No. 5,346,630 teaches a method of pressure spraying a solution of a dewatering aid from a position within the filter cake forming zone of a filter just prior to the disappearance of the supernatant process water. This method, which is referred to as torpedo-spray system, ensures even distribution of the dewatering aid without becoming significantly diluted by the supernatant process water.
It is well known that high HLB surfactants can actually cause an increase in moisture in dewatering hydrophobic materials such as coal. Due to the high polarity of its head group, high HLB surfactants adsorb on hydrophobic surfaces with inverse orientation, i.e., with hydrocarbon tails in contact with the surface and the polar heads pointing toward the aqueous phase. Such an adsorption mechanism should decrease the hydrophobicity and, hence, cause an increase in cake moisture. Most of the flocculants used as dewatering aids also dampen the hydrophobicity, and cause an increase in moisture.
There are several U.S. patents, which disclosed methods of using low HLB surfactants as dewatering aids. The U.S. Pat. Nos. 4,447,344 and 4,410,431 disclosed methods of using water insoluble nonionic surfactants with their HLB numbers in the range of 6 to 12. These reagents were used together with reagents (hydrotropes) that are capable of keeping the surfactants in solution or at the air-water interface rather than at the solid-liquid interface, so that they can be fully utilized in lowering surface tension. The advantage of using low HLB surfactants may be that unlike the high HLB surfactants they do not have the deleterious effects of hydrophobicity dampening.
The U.S. Pat. No. 5,670,056 teaches a method of using non-ionic low HLB surfactants and polymers as hydrophobizing agents that can increase the contact angle above 65° and, thereby, reduce the cake moisture. Monounsaturated fatty esters, fatty esters whose HLB numbers are less than 10, and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. The fatty esters were used with or without using butanol as a carrier solvent for the low-HLB surfactants. This invention disclosure lists a group of particulate materials that can be dewatered using these reagents. These include coals, clays, sulfide minerals, phosphates, metal oxide minerals, industrial minerals and waste materials, most of which are hydrophilic. The use of the low HLB surfactants disclosed in the U.S. Pat. No. 5,670,056 may be able to increase the contact angles of the materials that are already hydrophobic but not for the hydrophilic particles.
The U.S. Pat. No. 2,864,765 teaches a method of using a polyoxyethylene ehter of a hexitol anhydride partial long chain fatty acid ester, functioning alone or as a solution in kerosene. However, the disclosure does not mention that the nonionic surfactant increases the hydrophobicity of moderately hydrophobic particles. Furthermore, the compounds disclosed are essentially not adsorbed upon the solid surface of the ore particles and remain in the filtrate, as noted in the U.S. Pat. No. 4,156,649. In the latter patent and also in the U.S. Pat. No. 4,191,655, methods of using linear or branched alkylethoxylated alcohols as dewatering aids were disclosed. They were used in solutions of hydrocarbon solvents but in the presence of water-soluble emulsifiers such as sodium dioctylsulfosuccinate. As has already been discussed, the use of such a high HLB surfactant can dampen the hydrophobicity and cause an increase in moisture.
The U.S. Pat. No. 5,048,199 disclosed a method of using a mixture of a non-ionic surfactant, a sulfosuccinate, and a deforming agent. The U.S. Pat. No. 4,039,466 disclosed a method of using a combination of nonionic surfactant having a polyoxyalkylene group and an anionic surfactant. The U.S. Pat. No. 5,215,669 teaches a method of using water-soluble mixed hydroxyether, which is supposed to work well on both hydrophobic (coal) and hydrophilic (sewage sludge) materials. The U.S. Pat. No. 5,167,831 teaches methods of using non-ionic surfactants with HLB numbers of 10 to 14. This process is useful for dewatering Bayer process alumina trihydrate, which is hydrophilic. The U.S. Pat. No. 5,011,612 disclosed methods of using C
8
to C
20
fatty acids, fatty acid precursors such as esters or amides, or a fatty acid blend. Again, these reagents are designed to dewater hydrophilic alumina trihydrate.
The U.S. Pat. No. 4,206,063 teaches methods of using a polyethylene glycol ether of a linear glycol with its HLB number in the range of 10 to 15 and a linear primary alcohol ethoxylate containing 12 to 13 carbon atoms in the alkyl moiety. These reagents were
Grossman Tucker Perreault & Pfleger
Wilson Pamela
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