Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2000-03-17
2002-04-23
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S730000, C210S777000, C210S728000, C044S626000, C209S005000
Reexamination Certificate
active
06375853
ABSTRACT:
BACKGROUND
Solid-liquid separation is an important process in various industries. Water is often the liquid, as it is often the cheapest medium in which various industrial processes are carried out. The process of 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 increasingly difficult with decreasing particle size.
In a mechanical dewatering process, the particulate materials present in a feed slurry quickly form a bed (or cake) of particles on a medium before the water flows through the cake. The water flow rate, Q, through the cake is determined by the Darcy's law:
Q
=
K
Δ
PA
μ
L
[
1
]
where K is the permeability of the cake, &Dgr;P the pressure drop across the cake, A the filter area, &mgr; the dynamic viscosity of water, and L is the cake thickness. The driving force for the flow of water is the pressure drop. In vacuum filtration, the pressure drop is less than 1 atm, while in pressure filtration pressures as high as 8-10 atm are applied. In centrifugal filtration, the pressure drop is created by centrifugal force.
A filter cake is considered to consist of capillaries of various radii, which are determined by the size distribution of the particles constituting the filter cake. In a given capillary of radius, r, the water will flow through the capillary if the pressure drop, &Dgr;P, exceeds the pressure of the water inside the capillary. The capillary pressure, p, is given by the Laplace equation:
p
=
2
γ
cos
θ
r
,
[
2
]
where &ggr; is the surface tension of water and &thgr; is the contact angle of the particles in a filter cake. The contact angle is a measure of the hydrophobicity (water-hating property) of the particles.
Eq. [2] suggests three ways of achieving low cake moistures after filtration. These include i) surface tension lowering, ii) capillary radius enlargement, and iii) contact angle increase. Various chemicals (dewatering aids) are used to control these parameters. One group of reagents is the surfactants that can lower the surface tension. Most of the surface tension lowering agents used in industry are ionic surfactants with high hydrophile-lipophile balance (HLB) numbers, which tend to reduce contact angles and, hence, are detrimental to dewatering. Another group of reagents used as dewatering aids are inorganic electrolytes and organic polymers that are used as coagulants and flocculants, respectively. Both of these reagents are designed to increase the particles size and hence increase the capillary radius. However, they too tend to increase the contact angle of the particles, as they are hydrophilic in nature.
The U.S. Pat. No. 5,670,056 teaches a method of using non-ionic (or neutral) low HLB surfactants and water-soluble polymers as hydrophobizing agents that can increase the contact angle. Mono-unsaturated fatty esters, fatty esters and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. The fatty esters were used with or without using butanol as a carrier solvent. In a U.S. patent disclosure (Ser. No. 09/327,266), methods of using various other low HLB surfactants as dewatering aids are taught. In another U.S. patent application (Ser. No. 09/326,330), methods of using lipids are disclosed. The primary role of these reagents is to increase the contact angle of the particles to be dewatered. However, they also enlarge particles via hydrophobic coagulation and reduce surface tension. Thus, the dewatering aids disclosed in the pending applications addresses all of the three parameters, i.e., surface tension, contact angle, and capillary radius, toward the right direction. The net result of using such reagents as dewatering aids is that the rate of dewatering (given by Eq. [1]) is vastly higher than other dewatering aids, which gives rise to lower cake moistures.
An advantage of using the lipids as dewatering aids, as disclosed in U.S. patent application with a Ser. No. 09/326,330, is that they are of low cost and environmentally safe to use. Lipids are naturally occurring hydrophobic organic molecules isolated from biological cells and tissues. Animal fats and vegetable oils are the most widely occurring lipids, which are triesters of glycerol with three long-chain carboxylic acids. The performance of these reagents is slightly inferior to those of the low HLB: surfactants disclosed in the U.S. patent application with a Ser. No. 09/327,266, which may be attributed to the likelihood that the lipid molecules are too large to form close-packed monolayers of hydrophobes on the surface of the particles to be dewatered.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a novel method of decreasing the moisture of fine particulate materials during mechanical dewatering processes such as vacuum filtration, pressure filtration, and centrifugal filtration.
Another important object of the invention is to increase the rate at which water is removed so that given dewatering equipment can process higher tonnages of particulate materials.
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.
Yet another object of the invention is the provision of methods of controlling the frothing properties of the flotation product.
Perhaps the most important object of the instant invention is to achieve all of the above objects using low-cost affordable dewatering aids that have no harmful effects on the environment and the human health.
SUMMARY OF THE INVENTION
It is the most important object of this invention to provide an efficient method of dewatering fine particulate materials. This is achieved by destabilizing the water on the surface of the particles to be dewatered by rendering the surface substantially hydrophobic. The particles are hydrophobized normally in two steps. Initially, surfactants of high hydrophile-liphophile balance (HLB) numbers are used to render a particulate material moderately hydrophobic. The material is subsequently treated with a modified lipid to further enhance its hydrophobicity close to or above the water contact angle of 90°. This will greatly decrease the pressure of the water in the capillaries formed between the particles in a filter cake, and thereby allow the water to be removed more readily during mechanical dewatering processes.
A key to the methods of dewatering described in the present invention disclosure is the hydrophobicity enhancement step. According to the Laplace equation, a relatively small increment in hydrophobicity (above the level that can normally be achieved using a high HLB surfactant in the first hydrophobization step) can bring about a large decrease in the capillary pressure. The initial hydrophobization step may be omitted, if the particulate material is naturally hydrophobic or has been hydrophobized in an upstream process (e.g., flotation) preceding dewatering. However, the particles must remain reasonably hydrophobic at the time of the hydrophobicity enhancement step. Otherwise, the dewatering aids added in this step do not adsorb on the surface of the particulate material and fail to enhance its hydrophobicity.
In the present invention, naturally occurring lipids of vegetable and animal origin are broken into smaller molecules, so that they can more readily form close-packed layers of hydrophobes and, hence, greatly enhance the hydrophobicity of the particles. The lipid molecules are transesterified by reacting with alcohols in an appropriate catalyst to form monoesters, interesterified with glycerol to form mono- and diacylglycerols, and saponified and then acidulated to form fatty acids. The reaction products are used directly as dewatering aids without purification, which will
Grossman Tucker Perreault & Pfleger PLLC
Hruskoci Peter A.
LandOfFree
Methods of using modified natural products as dewatering... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods of using modified natural products as dewatering..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods of using modified natural products as dewatering... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2836018