GB1593805A - Flotation of oxidized coal with a latex emulsion of sodium polyacrylate - Google Patents

Description

(54) FLOTATION OF OXIDIZED COAL WITH A LATEX EMULSION OF SODIUM POLYACRYLATE (71) We, NALCO CHEMICAL COMPANY a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 2901 Butterfield Road, Oak Brook, Illinois, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of increasing the yield of oxidized coal where said coal or coal particles are subjected to concentrations by froth flotation and to the treating agent used in said method.

Coal generally is mined in this country and elsewhere from two different sources. A first source of great importance to retrieval of coal present is coal mined from so-called strip mines where the coal is near or at the surface of the ground and the veins are stripped therefrom. During this stripping process and before the coal is actually retrieved, the surface veins of coal are subjected to a significant amount of air oxidation which apparently changes the characteristics of the particles so that the results obtained in a concentration by froth flotation are different from concentration of the coal from the other source which is mined underground generally at a depth of greater than 100 feet from the surface and where there is less oxidation as in the underground mines of Pennsylvania and West Virginia.

The term "oxidized coal" in the present invention is defined as any type of weathered coal such as strip-mined coal or native or deep mined coal in which there has been a 1% or greater increase in oxygen content due to weathering, stock-piling, long storage times, etc.

The different degrees of weathering in coal seams as to oxygen content is highly variable and the following analysis is taken from Karaganda coals (Russian) and is cited from A.A.

Agroskin, Chemistry and Technology of Coal, 1961, page 33, translated by the Israel Program for Scientific Translations 1966: Carbon 77.9 - 88.3% Hydrogen 4.2 - 5.7% Nitrogen 1.0 - 1.7% Oxygen 5.2 - 16.2% The deleterious effect of an increase of oxygen in coal has been noted by several authors, e.g., S.C. Sun, Coal Preparation, "Part 3, Froth Flotation", page 10-67, "The unfloatability of oxygen and mineral matter is indicated by the nonfloatable lignite and animal charcoal.The deleterious effect of oxygen on the floatability of coals and coke has been described It is further noted that coal is readily oxidized in air and this process sometimes even gives rise to spontaneous combustion in the coal and results in weathering or loss of calorific value and coking power during storage in the open.

As is known, flotation is a process for separating finely ground mineral such as coal particles from their associate waste or gangue by means of the affinity of surfaces of these particles for air bubbles, which is a method for concentrating coal particles. In the flotation process a hydrophobic coating is placed on the particles which acts as a bridge so that the particles may attach to the air bubble and be floated, since the air bubble will not normally adhere to a clean mineral surface such as coal.

In froth flotation of coal a froth is formed as aforesaid by introducing air into a so-called pulp which contains the impure finely divided coal particles and water containing a frothing agent. The flotation separation of coal from the residue of gangue depends upon the relatively wettability of surfaces and the contact angle, which is the angle created by the solid air bubble interface.

In the development of flotation to date, three general classes of reagents have been utilized: (1) collectors or promoters, (2) modifiers, and (3) frothers.

The collectors may be selected from such compounds, among others, as primary amines, quaternary ammonium salts, xanthates, fatty acid soaps, alkyl sulfates, etc. A typical listing of commercial collectors is given in Kirk-Othmer, Encyclopedia of Chemical Technology, II, Vol. 9, page 384, Table 2.

Modifiers are such regulating agents as pH regulators, activators, depressants, dispersants, and flocculants.

A frothing agent is utilized to provide a stable flotation froth persistent enough to facilitate the coal separation but not so persistent that it cannot be broken to allow subsequent handling. Examples of commonly used frothing agents are pine oil, creosote, cresylic acid, and alcohols such as 4-methyl-2-pentanol. Alcohol frothers are preferred in the present invention and additional alcohols are illustrated by amyl and butyl alcohols, terpenol and cresols. An additional preferred alcohol is methyl isobutylcarbinol (MIBC), which is an aliphatic alcohol in common use as a frother.

The present treating agents which are water-soluble polyacrylates are useful as promoters and frothing aids.

A. Utilization of water-soluble polymers: U.S. 2,740,522 Aimone et al - The patentee utilizes water-soluble polymers in amounts .001 pound/ton to 1.0 pound/ton with a preferred amount of 0.01 pound/ton to 0.2 pound/ton. Example 16 (column 7) shows the flotation of Pennsylvania anthracite coal fines conditioned with 0.2 pound/ton of the sodium salt of hydrolyzed polyacrylonitrile to produce a rougher concentrate. A second portion of the example utilizes 0.5 pound/ton of polymer. This patent appears equivalent to British Patent 749,213.

B. Concentration of coal by flotation: U.S. 3,696,923 Miller In the above prior art, none of the patents noted dealt with the problems envisaged with the attempts to use flotation concentration on oxidized coal.

It was found that in attempting to float oxidized coal there were serious problems of flooding, stoppages of equipment, and unsatisfactory yield and this was true where a majority blend of deep mine coal was mixed with strip coal where 80% deep mine coal was utilized in the mixture.

The present invention provides a method of increasing the yield of oxidized coal undergoing a concentration treatment of froth flotation by using as a flotation promoter an invertible water-in-oil emulsion of sodium polyacrylate in a dosage calculated as 0.017 - 0.5 pound of dry sodium polyacrylate per ton of dry coal.

The present invention also provides a treating agent for oxidized coal undergoing froth flotation which comprises an invertible water-in-oil emulsion of sodium polyacrylate conforming to the following formula: a) from 5-29.75% by weight of the emulsion of an aliphatic hydrocarbon liquid; b) from 70-94.75% by weight of the emulsion of an aqueous phase consisting of water and from between 10-50% by weight of the emulsion of water-soluble sodium polyacrylate; c) from about 0.25-10% by weight of hydrophobic surfactants capable of stabilizing the emulsion.

The treating agent for the present invention may be defined as a promoter or frothing agent which is a latex or water-in-oil emulsion of a water-soluble anionic linear addition polymer of a polymerizable monoethylinically unsaturated compound having an average molecular weight of about 100,000 to 1,000,000 and more, with a preferred molecular weight of about 1,000,000 or more.

The promoter or frothing aid used in this invention is sodium polyacrylate. The dosage of this latter treating agent is in the range of 0.05 - 1.5 pounds of sodium polyacrylate latex per ton of dry coal (0.017 - 0.5 pound of dry sodium polyacrylate per ton of dry coal) and it is utilized conventionally as a 0.5-2% solution. Utilization has resulted in a 64.6% coal recovery as opposed to 16.4% recovery when using the dry polymer precipitate of sodium polyacrylate. It is noted in comparing Examples 1 and 2. post, that the recovery percent for polyacrylate utilized in emulsion form was 64.6% as compared with a lower form for sodium polyacrylate utilized in solution form of 20-30%.

Also operable in the present invention, together with the anionic sodium polyacrylate, are minor percentages of the non-anionic sodium polyacrylamide in the form of a mixture or copolymer wherein the percentile of polyacrylamide is up to 25% of the total. Such addition of polyacrylamide does not modify the basic anionic character of the polymer which is a necessary criteria.

Oxidized coal recovery utilizing sodium polyacrylate latex emulsion (oil-in-water) is shown in Table 1.

TABLE 1 Oxidized coal flotation using latex polymers Dosage (pounds/ton) Equivalent % Coal Promoter Effective to Latex Recovery 1) Sodium polyacrylate latex emulsion 0.3 0.3 64.6 2) Sodium polyacrylate dry 0.1 0.3 16.4 2a) Sodium polyacrylate dry 1.0 3.0 36.6 3) Azo-bis-isobutyro nitrile 0.006 0.3 37.3 3a) Azo-bis-isobutyro nitrile 0.06 3.0 34.5 4) Sorbitan monooleate (SPAN 80, ICI) 0.0066 0.3 20.1 4a) Sorbitan monooleate (SPAN 80, ICI) 0.066 3.0 32.2 In Table 1 above, Promoter No. 1 showed 64.6% coal recovery and included the reaction to the latex emulsion containing sodium polyacrylate and ingredients 3 and 4. The individual effect of the dry precipitate 2 at 16.4 and 36.6, depending on concentration, are also given. The individual effect of the azo initiator and the emulsifier (SPAN 80) (Registered Trade Mark) are set out.

TABLE 2 Comparative activity of the promoter with latex Polymers Dosage Dosage Run (pounds/ (pounds/ No. Frother ton) Promoter ton) Recovery* 1 MIBC** 0.2 LOPS(3 0 0 2 " 0.2 " 0.11 2.6 3 " 0.2 " 0.27 3.0 4 " 0.2 " 0.48 6.3 5 " 0.2 " 0.74 10.6 6 " 0.2 #2 Fuel Oil 0.80 44.9 7 " 0.2 " 0.50 18.7 8 " 0.2 " 0.30 7.0 9 " 0.2 Latex 0.20 10.7 Polymer A2 10 " 0.2 " 0.30 15.1 11 " 0.2 " 0.40 18.0 12 " 0.2 Latex Polymer 2 (0.30 40.9 & Fuel Oil (0.50 * % Recovery is on total solids, not actual coal in float ** Methyl isobutylcarbinol # Low odor paraffin solvent Coal:Oxidized coal from King Broellton Coal Company Polymer A2 is derived from monomer starting material A at page 8 post.

It is noted that, with reference to Runs 2 (utilizing LOPS) and 11 (utilizing the latex emulsion polymer) and considering that the amount of LOPS in the starting material is in the range 20-30%, the activity indicated in the percent recovery of coal shows a distinct increase of from 2.6 to 18.0. Again, with reference to Runs 3 and 8, the results for percent recovery would indicate that there is a similar activity in the use of LOPS and #2 fuel oil.

The preparation of the water-in-oil latex from monomers, its polymerization to a water-in-oil emulsion, and its subsequent inversion to an oil-in-water emulsion in use are described in one or more of the following patents: U.S. 3,997,429 Kane et al U.S. 3,624,019 Anderson et al U.S. 3,734,873 Anderson et al U.S. 3,826,771 Anderson et al A monomer starting material useful for forthing oxidized coal has a composition as follows:: Water 27.0 Caustic soda (50%) 23.0 Acid acrylic glacial 20.9 Low odor paraffin solvent (LOPS) 19.3 Sorbitan monooleate (SPAN 80 (Registered Trade Mark), ICI) 1.0 Azo-bis-isobutyronitrile (catalyst) 0.03 Espesol 3-E (Registered Trade Mark) (a liquid aromatic hydrocarbon blend, Charter International) 8.5 Polyisobutylene (stabilizer) 0.27 Aluminum tristearate (stabilizer) 0.0002 The polymerized sodium polyacrylate may be produced by polymerization of, for example, the above recipe according to the teachings of U.S. 3,284,393 Vanderhoff et al using a free radical type catalyst.

A typical water-in-oil polymeric emulsion contains 1) from between 10-29.75% by weight of sodium polyacrylate 2) from 5-94.75% by weight of the emulsion of an aliphatic hydrocarbon liquid 3) from 70-95% by weight of the emulsion of an aqueous phase consisting of water and polyacrylate (step 1) 4) from about 0.25-10.0% by weight of hydrophobic surfactants capable of forming a stable emulsion of the monomers (e.g., alkylated hydrocarbons such as toluene and xylene).

The above-noted composition is not self inverting but a variety of inverting techniques are set out in U.S. 3,624,019 (supra) at column 3, lines 49-57. The presence of any of a group of oil-in-water activators will cause the polymer emulsion to self invert. Such activators may be selected from: 1) Surfonic N-95 (Jefferson Chemical Co.), a nonylphenol with 10 moles ethylene oxide 2) Triton N-101 (Rohm & Haas), nonylphenoxy polyethoxyethanol 3) Makon 10 (Stepan Chemical Co.), alkyl phenoxy polyoxyethylene ethanol 4) Igepal CO 630 (GAF), nonylphenoxy poly(ethyleneoxy)ethanol.

In the present case the activator may be placed in a separate vehicle with water.

Alternatively, an activator may be added later to the polymerized composition for a self invert mode. An explanation of the action of the inversion technique is that a normal latex will generally be added to water containing a hydrophilic surfactant, as, for example, Surfonic N95, thereby causing the emulsion to invert and allowing the polymer previously in the discontinuous phase to wind up in the continuous phase of the water-in-oil emulsion.

This, of course, allows the polymer to solubilize. For self-inverting emulsions, the same original emulsion is carefully balanced so that, when added to water, the emulsion inverts, thereby allowing the polymer to solubilize.

Example I A sodium polyacrylate latex emulsion was fed into the flotation cell feed carrying oxidized coal. The latex promoted the flotation of fine coal resulting in increased fine coal recovery up to and including a 64% recovery rate. This sodium polyacrylate latex emulsion coal promoter thus proved effective in increasing recovery of oxidized coal. In use, dosage rates of the sodium polyacrylate latex emulsion varied from approximately 0.3 - 1.5 pounds of sodium polyacrylate latex per ton of dry coal fed to the flotation circuit. The latex was used in conjunction with a straight chain alcohol frother of the C6-C12 type. The alcohol frother dosage was approximately 0.15 pound/ton of dry coal feed. The frother was normally fed to the flotation cell head box.

Example 2 Comparative, using polyacrylate solution In 1976 a coal flotation promoter evaluation was run using sodium polyacrylate at an eastern U.S. coal preparation plant. This plant processes both deep mine and oxidized strip mine coal. Due to the difficulty in efficiently floating oxidized coal in the frother support, this company must feed a blend of these coals in which the oxidized constituent makes up only 10-20% of the total input. Primary difficulty in processing oxidized coal is that in the froth banks the coarser portion of the oxidized size (probably 100 x 28 mesh) does not readily float and consequently, the majority of oxidized coal particles in the froth are very fine.When this froth is fed to vacuum disk filters, where the clean coal is recovered, the high proportion of oxidized 0 x 100 mesh, coal particles blinds off the filters and reduces cake thickness and ultimate recovery. Further, this situation causes a significant increase in filter overflow, which is fed to a filter sump. Normally, when oxidized coal feed is increased to about 20% of the plant input for about an hour, the attendant reduction in vacuum filter efficiency causes the filter sump to become swamped to a point where it overflows onto the plant floor. This condition is further aggravated by the physical nature of the froth. When the oxidized coal feed is high, the float bubbles become large and extremely stable, resulting in a foamy mass having such integrity that it remains intact on the surface of the filtration tank.Ultimately it is discharged onto the plant floor when the filter sump overflows. In this situation, plant procedure is to reduce or shut down all oxidized coal until filter efficiency can be improved by running on a 90% to 100% deep mine input. Fairly often, the above-mentioned upset is so severe that total coal feed to the plant must be completely shut down until filter operation returns to normal.

A further difficulty is due to the low level of oxidized coal utilization; the plant depletes its deep mine coal stocks approximately every five or six hours. This requires the plant to shut down until sufficient deep mine coal is received for another five or six hour period of operation. This down time delay typically lasts from two to four hours.

Sodium polyacrylate was added to the slurry launder prior to the distribution box which feeds the nine flotation banks. The feed slurry of -28 mesh material came from a series of sieve bands which all discharge into the common launder. The sodium polyacrylate was added at a point of high turbulence. The treated feed slurry drops by gravity into the distribution box which also exhibits high turbulence.

Conclusions Referring to Table 3, it is noted that set 2 versus set 1 indicates a 23.5% increase flotation solids recovery where the sodium polyacrylate was utilized at dosages of 0.075 to 0.088 pound/ton of coal. The procedure at the coal plant was to utilize a standard collector and frother and process line similar to that taught in U.S. Patent 3,696,923 Miller. A subsequent comparison of set 5 against set 4 did not produce a difference in percent solids. However, the percent solid input in 4 is higher and the set 5 feed ash is greater than for set 4. It is noted that, in the operating procedure to obtain the results in sample set 5, these were taken after severe plant upsets had occurred where maximum float quality and filter efficiency could not be re-established. Additional examples not in the table showed that the increase in recovery of the float solids varies from about 20-30% or 23-30%. 8ABLE 3 Summary of results on composite samples collected Sodium Feed Float Tails Sample Polyacrylate % % % Set Frother Dosage Dosage Solids % Coal % Ash Solids % Coal % Ash Solids % Coal % Ash No. 1 0.04 pound/ton 6.46 84.05 15.95 24.37 94.88 5.12 1.21 42.99 57.01 No. 2 0.04 pound/ton 0.075-0.088 6.31 83.62 16.38 30.10 94.45 5.55 1.31 41.29 58.71 pounds/ton No. 3 Not used because of plant shut down No. 4 0.12 pound/ton 7.81 84.87 15.13 25.89 94.18 5.82 1.35 44.00 56.00 No. 5 0.10 pound/ton 0.032 6.26 83.23 16.77 23.99 93.21 6.79 1.17 35.66 64.34

Claims (7)

WHAT WE CLAIM IS:

1. A method for the concentration of oxidized coal by froth flotation by using as a flotation promoter an invertible water-in-oil emulsion of sodium polyacrylate in a dosage calculated as 0.017 - 0.5 pound of dry sodium polyacrylate per ton of dry coal.

2. The method according to claim 1 wherein the water-in-oil emulsion contains sodium polyacrylate, a paraffinic solvent, a water-in-oil emulsifier, an oil-in-water activator as hereinbefore defined, and a minor amount of stabilizers.

3. The method according to claim 2 wherein the oil-in-water activator is added separately.

4. The method according to claim 1, 2 or 3 wherein the water-in-oil sodium polyacrylate emulsion inverts on usage and contact with water to an oil-in-water emulsion.

5. A treating agent for oxidized coal undergoing froth flotation which comprises an invertible water-in-oil emulsion of sodium polyacrylate conforming to the following formula: a) from 5-29.75% by weight of the emulsion of an aliphatic hydrocarbon liquid; b) from 70-94.75% by weight of the emulsion of an aqueous phase consisting of water and from between 10-50% by weight of the emulsion of water-soluble sodium polyacrylate; c) from about 0.25 - 10% by weight of hydrophobic surfactants capable of stabilizing the emulsion.

6. A method for the concentration of oxidized coal undergoing a concentration treatment of froth flotation substantially as herein described.

7. A treating agent for oxidized coal undergoing froth flotation substantially as herein described.