GB2164271A - Process for froth flotation of fossilized organic mineral values - Google Patents

Abstract

In a method for beneficiating coal or kerogen-containing oil shale values by froth flotation, improved recoveries and improved grades of the organic mineral values are obtained by first treating the ores containing them in an attrition milling apparatus to liberate the organic values from associated inorganic impurities. Attrition milling also provides special surface characteristics to the liberated organic values rendering them specially adapted for beneficiation by fine particle flotation. In accordance with the process, liberation is achieved by attrition milling for a time sufficient to provide particles of from about 1 to about 10 microns.

Description

SPECIFICATION Process for improved flotation of coal and kerogen-containing oil shale Background of the invention The present invention relates to an improved process for beneficiating and recovering coal and kerogen-containing materials by froth flotation. More particularly, it relates to a froth flotation process for coal and oil shale, wherein the coal and/or oil shale are finely attritioned in an attrition mill prior to froth flotation to provide increased overall kerogen and coal recovery and improved grades.

Froth flotation is one of the most widely used methods for beneficiating mineral values from ores and mineral deposits containing them. It is especially used for separating finely ground valuable minerals from their associated gangue or for separating valuable minerals from one another. The process is based on the affinity of suitably prepared mineral surfaces for air bubbles. In froth flotation, a froth or a foam is formed by introducing air into an agitated pulp of the finely ground ore in water containing the frothing or foaming agent. A chief advantage of separation by froth flotation is that it is a relatively efficient operation involving substantially lower costs than many other processes.

Current theory and practice state that the success of a froth flotation process depends to a great degree on reagents called collectors that impart selective hydrophobicity to the mineral value that has to be separated from other minerals or gangue materials. Thus, the flotation separation of one mineral species from another depends upon the relative wettability of mineral surfaces by water. Typically, the surface free energy is reportedly lowered by the adsorption of heteropolar collectors. The hydrophobic coating thus provided acts in this explanation as a bridge so that the mineral particles may be attached to an air bubble. The practice of this invention is not, however, limited by this or other theories of flotation.

Coal is the largest available fossil fuel energy source known today and both coal and kerogen-containing oil shale are becoming an important energy source for the future in view of the continuing depletion of crude oil. Coal and oil shale, as obtained from mining operations, are recovered in a variety of particle sizes contaminated with clay and other inorganic impurities. Some clay and inorganic impurities may be readily washed from the surfaces of the larger coal and oil shale particles, but a significant quantity of bound impurities still remains in the larger washed particles. More-over, a significant quantity of coal and kerogen-containing oil shale values of small particle size are present in the wash residues, said residues commonly being referred to as clay slimes.Coal and kerogen values are conventionally recovered from clay slimes by froth flotation methods, using a frothing agent generally comprising a low molecular weight aliphatic alcohol and as the collector a hydrocarbon oil is conventionally used, such as kerosene, diesel or fuel oils.

Prior art methods for improving the efficiency of coal flotation operations have concentrated on varying the type of frother and or collector used. In U.S. 3,458,044, for example, a highly volatile organic liquid is used as the frothericarrier instead of air and a low volatility hydrocarbon oil is employed as the collector with flotation being carried out under reduced pressure. This patent discloses that good recoveries are obtained with slurries containing average coal particle sizes of from 50 to less than about 1000 microns and preferably in the range of from 50 to 400 microns. In U.S. 4,272,364 it is disclosed that improved coal recoveries are obtained by substituting 4,4-dimethyl-1-pentanol as the frothing agent in place of 4-methyl-2-pentanol (methyl isobutylcarbinol, MIBC), the conventional frothing agent. In U.S.

4,416,769, it is disclosed that in coal flotation, improved recoveries at improved grades are obtained by substituting a pariffinic residual oil having a cut point at atmospheric pressure of at least 190 C, such as a coal spray oil of the type used in coke oven technology, in place of the conventional distilled hydrocarbon oil collectors, together with a polyglycol ether type frothing agent. Other methods for improving the recovery and grade of coal obtained by froth flotation have included the use of selective depressants or flocculants, to selectively depress the clay slimes and thereby selectively promote the flotation of a purer coal froth, as is, for example, illustrated in U.S. 4,268,379 wherein certain cationic polymers are disclosed for this purpose.

These prior art methods have generally improved the quantities of coal recovered by froth flotation methods. By 1981, it was estimated that over 45 million tons of coal had been recovered by froth flotation. In spite of the huge quantities of coal recovered by conventional flotation procedures, a significant quantity of coal and kerogen-containing oil shale values are not recovered, which represents an unnecessary loss of energy source. These unnecessary losses are due in large part to high inorganic values associated with the ultrafine particles and high inherent inorganic matter associated with some coals, ie., where the particle size becomes too small, the use of flotation to obtain enough coal of a satisfactory grade becomes economically unattractive.Thus, in conventional processes, aqueous slurries of coal particles are classified, and slurries containing particles having an average particle size of 50 microns or larger, sometimes as low as 20 microns, are retained for flotation recovery efforts, whereas slurries containing particles of below 50 or 20 micron sizes are generally discarded as unrecoverable waste. The larger particle-sized coal recovered, however, generally contains significant amounts of bound inorganic impurities, commonly referred to as ash, which are not separated from the organic coal values in these flotation methods. Another shortcoming of conventional flotation methods is that a significant amount of coal values are discharged as waste, which still represents a loss of a precious resource for energy pro duction.

In order to improve upon the shortcomings of the prior art processes, it is an object of the present invention to provide a new and improved froth flotation process for fossilized organic minerals capable of yielding improved recovery at improved grade.

It is another object of the present invention to provide a new and improved froth flotation process which permits recovery of finely divided coal and kerogen-containing oil shale having an average particle size of less than about 20 microns.

It is a further object of the present invention to provide a new and improved flotation process for recovering and or beneficiating coal and or kerogen containing oil shale which is more economical in terms of time as well as energy consumed.

Summary of the invention In accordance with these and other objects, the present invention provides a new and improved process for beneficiating finely divided coal and'our kerogen-containing oil shale values from a mineral deposit or ore containing these fossilized organic values and associated inorganic impurities, said process comprising:: (a) providing a slurry of finely divided particles of said ore in an aqueous medium; (b) treating the particles in said slurry by attrition milling for a time sufficient to provide an aqueous slurry of attritioned, liberated coal and our kepgen-containing oil shale particles having an average particle size of from about 1 to about 10 microns; (c) conditioning said attritioned slurry with effective amounts of a frothing agent and a collector, respectively; and (d) thereafter, collecting the coal and our keiogen-containing oil shale values by froth flotation procedures.

In accordance with the present invention, it has unexpectedly been discovered that improved overall coal and or kerogen-containing oil shale recoveries at improved grades are obtained in a froth flotation process by decreasing the average particle size of mineral deposits or ores containing these fossilized organic minerals to about 10 microns or less, and preferably to about 5 microns, by attritioning methods.

It has also been unexpectedly and surprisingly discovered that attrition milling of the ores to the particle sizes specified provides special surface characteristics to the fossilized organic mineral values rendering them especially suited for selective recovery in froth flotation procedures.

In accordance with the present process, mineral deposits or ores containing fossilized organic minerals, such as coal and kerogen-containing oil shale, are size-reduced by conventional methods to provide a slurry of ore particles of a particle size suitable as a feed for an attrition milling apparatus. The ore particles are treated in the attrition mill to further size reduce the ore particles for a time sufficient to liberate the coal and or kerogen mineral values from the associated inorganic impurities and to provide special surface characteristics to the value organics, thereby improving the results obtained in the subsequent froth flotation steps.The slurry containing finely attritioned, liberation-sized ore particles is then conditioned with effective amounts of a frothing agent and collector, and any conventional frother and collector may be used herein.

The conditioned slurry of attrition milled, liberation-sized particles is thereafter floated in accordance with conventional methods, e.g., in a flotation cell wherein air is introduced to the conditioned slurry at a specified rate while the slurry is agitated to produce a surface froth rich in coal and/or kerogen-containing oil shale values. The organics rich froth is collected by conventional methods to yield a concentrate and unfloated slurry materials known as tailings. The pulp slurry may be subjected to a second stage flotation by stopping the flow of air bubbles, reconditioning the slurry under agitation with additional frother and collector, followed by a second flotation step.The organics rich concentrate may thereafter be used as is, or may be subjected to further purification by cleaner flotation methods, wherein the concentrate collected is in successively cleaner stages re-slurried, re-ground, re-conditioned and floated again to form cleaner concentrates and tailings. Conventional floc-culating agents may also be added if desired.

In the attrition milling treating step of the present process, the ore particles are attrition milled to provide liberation-sized particles having a surface adapted to improved flotation, by attrition milling for a time sufficient to provide ore particles having an average particle size of from about 1 to about 10 microns, inclusive. The grinding media employed in the attrition mill is not critical, so long as the grinding media employed provides the liberation-sized ore particles having attrition milled surface characteristics, i.e. the grinding media must be effective to grind the ore to the particle size range given above. Generally, in an attrition mill, the ore can be finely attritioned to libera-tion size by grinding for a period of from 1 minute to about 1 hour, usually for less than 30 minutes and often for less than 15 minutes.

Attrition milling also provides a distinct advantage over other size reducing methods, such as ball milling or pebble milling, because the time (and therefore energy) required for ball or pebble milling to provide particles having an average particle size of from about 1 to about 10 microns is at least hours instead of minutes.

The new and improved process of the present invention appears to be the first feasible mineral processing method for the removal of inherent inorganic impurities from coal and oil shale. The present proc ess is useful to upgrade coal and oil shale mineral deposits, feedstocks, slurries and the like to provide, for example, high grade feed for coal liquefaction plants; high grade coal water slurries; upgrading of low grade, i.e., high ash content coals; as well as to provide kerogen-containing oil shale suitable for retort feedstock. The present process also provides an efficient economical method for recovering finely divided fossilized organic mineral values, which were heretofore discarded as unusable waste.

Other objects and advantages of the present invention will become apparent from the following detailed description in conjunction with the illustrative working examples.

Detailed description of the invention The process of the present invention provides a new and improved method for beneficiating fossilized organic minerals from associated inorganic impurities by froth flotation methods which comprises a two fold size-reduction method followed by fine particle flotation.

More particularly, in accordance with the present process, an organic mineral containing ore or other low grade organic mineral feedstock is first size-reduced by conventional methods to provide a feed suitable for attrition milling. Generally, and without limitation, a feed suitable for attrition milling is comprised of particles having an average particle size of from about 0.5 to about 5.0 mm, and preferably about 2.0 mm. Illustrative methods for size-reducing the coal or oil shale to provide a feed suitable for attrition milling include, for example, hammer milling, roll crushing, rotary or gyratory crushing, steel ball milling and pebble milling, although any method known to be useful for size reducing ores to an average particle size of from about 0.5 to 5.0 mm and preferably about 1 to 3 mm may be used.The resulting size-reduced feed particles, however, still contain substantial amounts of associated bound inorganic impurities. These inorganic materials remaining as impurities in the crushed or ground coal and oil shale are effectively separated or liberated in the subsequent attrition milling operation.

In accordance with the present process, the crushed or ground ore feed is introduced to an attrition milling apparatus, wherein the feed is finely attritioned to an average particle size of from about 1 to 10 microns, to liberate the fossilized organic values from inorganic impurities and to provide attrition-milled surface characteristics to the particular organic values in the slurry. By an attrition mill is meant an apparatus capable of producing finely divided particles of 1 to 10 microns average particle size by a scrubbing or abrading motion or method, as opposed to an impact motion or method.Impact mills such as steel ball mills, hammer mills and the like mentioned above for use in the primary size-reduction step are not included herein because, even if those mills capable of size reducing the ore to an average particle size of 1 to 10 microns are used, they do not provide the special surface characteristics to the finely divided ore particles, as are provided by an attrition mill. The reason why impact milling does not provide the ore particles with the same surface characteristics, and hence flotation response, as attrition milling is not completely understood. It may be that impact mills cause a certain flattening of the ore particles, or may cause particle agglomeration to occur.It may also be that, for example in the case of a steel ball mill, the steel ball grinding media deposit iron values on the surfaces of the organic particles which interfere with adsorption of the organic collectors, i.e., blind the collectors with respect to these organic particles. Without wishing to be bound by any particular theory, for whatever the reason, applicants have determined that impact-type mills do not provide the desired surface characteristics to liberation-sized ore particles, whereas attrition mills do provide these special surface characteristics.

Illustrative attrition mills which are effective to size-reduce fossilized organic particles by scrubbing or abrasion to an average particle size of about 1 to about 10 microns include both stirred and vibratory attrition mills. Examples of stirred attrition mills include, for example, sandmills which provide a tumbling motion on an eccentric in the presence of grinding media to scrub and abrade the particles. Other stirred type attrition mills contain an internal stirring mechanism capable of abrading and scrubbing the organic mineral particles with the grinding media. Suitable stirred-type attrition mills which are available commercially include for example, SUPERMILL- from Premier Milling Company, AiTRITOR from Union Processing Company, as well as stirred ball mills available from a number of sources, to name a few.

Vibratory attrition mills are also commerciallv available for example, VIBRO-ENERGY Mill from SWECO Corporation, VIBRATRON- from Schutz-Oneill Company and VIBRATORY MILL from Allis-Chalmers, to name but a few. A preferred attrition mill for use in the process of the present invention comprises a sandmilling apparatus.

The grinding media employed in the attriti )n milling apparatus is generally not critical and any grinding medium may be employed so long as it s effective to reduce the average particle size of the ore to between about 1 and 10 microns in a period of from 1 to 30 minutes. Illustrative grinding media for use herein include finely divided hard particles s-ich as, for example, graded sand, glass beads, polymer beads, metal shot, ceramics such as titania c - alumina and the like. Autogenous grinding may also be performed in the attrition mill with or without t grinding media. A preferred grinding medium for use in a sandmill attritioning apparatus in accordanc' with the present invent on comprises 3 mm glass beads, although grinding media of different sizes may be used. The amount of grinding media added to the attrition mill may vary broadly, however effi .ient attrition milling has been achieved by adding 2 kilograms of grinding medium per 100 gms of c e, dry weight basis. Typically, the attrition milling feed will be in the form of an aqueous slurry containiiig from about 10 to about 90% by weight 0.5 to 5.0 mm solids, and preferably 40% to 50% by weight solids.

In accordance with the present process, the attrition milled slurry is conditioned with effective amounts of a frothing agent and collector, respectively. As is readily apparent to those skilled in this art, the slurry of ore particles may be conditioned with frother and collector before or during the first size-reducing stage to provide the feed for the attrition mill; before or during the attrition milling step; as well as in a flotation cell. The srage at which the pulp slurry of ore particles is conditioned is generally not critical to this invention, as Ic ng as the slurry solids are effectively conditioned prior to the floating step.

The frothing ageiit is used to provide a stable flotation froth which is durable enough to facilitate separation of the floated fossilized organic values but not so durable that it cannot be broken to permit subsequent handling. Suitable frothing agents for use herein comprise pine oil, creosote, cresylic acid, C4 to C12 alcohols such as methylisobutylcarbinol (MIBC) (i.e., 4-methyl-2-pentanol), as well as polyols, such as polypropylene glycols and polyoxy-alkylene glycols. The frother is generally added in an amount of from about 0.01 to about 2.0 pounds per ton of fine coal.

The collector is added to provide a hydrophobic. air-avid adsorbed coating to the surface of the attrition milled coal or kerogen-containing oil shale particles and to prevent re-wetting of the particles by water once they are floated into the flotation froth. The collectors for use in the present invention comprise hydrocarbon oils selected from kerosene, gas oils, residual fuel oils, phenol extracts and coal tars.

The collector employed will generally be added in an amount of from about 0.1% to about 10% by weight based on dry ore.

The slurry of ore particles in aqueous media is conditioned by adding frother and collector and agitating the slurry for a time sufficient to distribu e the frother and collector throughout. Generally, the slurry is conditioned for a period of about 0.5 to at out 10.0 minutes during agitation at from about 500 to 2000 rpm.

If desired, modifiers such as conditioning urfactants such as the bisalkyl esters of a sulfosuccinic acid salts described in U.S. 4,196,092, incorporatt d herein by reference, or flocculants, such as the cationic polyamine flocculants described in U.S. 4,26 3,379, also incorporated herein by reference, may be added during the conditioning step in their conventionally added amounts.

After the attrition milled slurry is properly conditioned, it is subjected to conventional froth flotation procedures. In such procedure, air bubbles are introduced into the attrition milled slurry which is maintained under constant agitation to form a froth on the surface of the slurry. The air bubbles attach to the attrition milled fossilized organic mineral value particles and cause them to levitate and become part of the froth. The desired organic values, i.e., the liberated coal andíor kerogen-containing oil shale, are isolated from the other ingredients in the slurry by any suitable manner, such as by skimming or scooping the froth or by permitting the froth to spill over a weir, followed by draining on a screen or by suction on a filter.Generally and without limitation, the flotation is performed by agitating the conditioned attrition milled slurry at from about 1,000 to 2,000 rpm while introducing a flow of air at a rate of from about 2 to about 10 litreminute for a period of from about 1 to about 10 minutes with frequent repeated skimming of the froth. The higher grade fossilized organic products obtained may be used as is or refloated to provide cleaner concentrates.

The process of the present invention provides the first feasible means for satisfactorily floating finely divided coal and or kerogen-containing oil shale of minute particle size. It also provides the first feasible method for removing the inherent inorganic materials from coal and oil shale, by permitting satisfactory flotation of liberation-sized particles of the fossilized organic mineral values. The improved flotation recoveries obtained with the process of the present invention are believed to be due, in part, because of special surface characteristics imparted to the organics values by attrition milling, which are not exhibited by similar particle sized organic particles prepared by other size-reduction methods.

Other objects and advantages of the present invention will become apparent from the following working Examples, which are provided by way of further illustration only to enable those skilled in this art to better understand and practice the present invention.

Examples 1-3 In each of the following Examples, the following preparation and testing procedures were used: 250 grams of a coal containing about 25% inherent ash content were crushed in a 6 inch diameter hammer mill to provide a coal feed having an average particle size of 0.125 mm, suitable as a feed for an attrition mill (sandmill). For comparison, in some of the Examples, the ores were further size reduced in an 8 inch diameter steel ball mill containing a steel ball charge of 5.3 kg at 50% solids and in others an 8 inch diameter pebble mill was used containing a 3.0 kg pebble charge at 50% solids. The attrition mill used was a 4.5 inch diameter sandmill containing a 1.0 kg charge of 3 mm glass beads at 50% solids.

The second stage size-reductions were carried out in the various mills for the times indicated to provide the particle sizes shown.

The milled slurry was transferred to a Denver D12 rectangular flotation cell. The volume of the slurry was adjusted to 2160 mis by adding water to provide a pulp density of about 4.5% solids and a slurry level in the cell at about 2 cm below the lip.

Collector and frother were added to the slurry while the slurry was agitated at about 1200 rpm. The collector used was a No. 2 fuel oil. The frother used was a 2-ethylhexanol.

The slurry was conditioned for a period of 2 minutes. At the end of the two minutes conditioning, air was fed at about 6 liters minute from a compressed air cylinder. The froth flotation was continued for about 3 minutes during which the concentrate was collected. The flotation times were predetermined to give a barren froth upon completion of flotation.

All flotation products were filtered, dried and analyzed for ash content, i.e., inorganic impurities. Deionized water was used in all tests.

The results obtained are set forth in Table 1 as follows: TABLE 1 Fine coal flotation of high inherent ash content coal Frother dosage 1.0 Ibslton; Collector dosage 1.0 Ibsiton Average Particle Milling Milling Size, Coal Grade, Example apparatus Time Microns Recovery, 0/c % Ash A. Steel Ball 2.5 hrs. 4.8 '3.3 11.5 Mill B. Pebble Mill 30 min. 24.2 83.2 16.6 C. Pebble Mill 1.0 hr. 11.7 63.4 10.9 D. Pebble Mill 2.5 hrs. 10.5 56.0 7.9 1. Attrition 3 min. 8.8 83.3 12.4 Mill 2. Attrition 7 min. 5.9 7Z.4 10.0 Mill 3.Attrition 9 min. 5.1 68.0 8.9 Mill The results of Table 1 clearly show that the coal that was sandmilled in accordance with the present invention, Examples 1-3, provided consistently better recovery grade values at much lower grind times, than the steel ball milled coal of Example A or the pebble milled coal of Examples B-D. The results show that smaller particle sizes produce flotation concentrates of better grade (lower ash content) and that only the sandmilled products provided higher recoveries at these grades. Sandmilling grind times were much faster than other conventional mills and produced truly liberation-sized particles in shorter times.The results therefore demonstrate that ore processing may be performed in accordance with the process of the present invention at a savings in energy consumption and also, more importantly, plant through-put is increased.

Example 4 In the following Example, identical preparation and flotation methods were used as in Examples 1-3 with the exception that 100 grams of an oil s!1ale obtained from the Green River formation in Colorado were used as the fossilized organic mineral starting material. The flotation performance characteristics of both a pebble-milled and a sandmilled ore were compared as in Examples 1-3. The results obtained are set forth in Table 2 as follows: TABLE 2 Oil Shale Flotation Average Particle Milling Milling Size Oil Shale Grade, Example Apparatus Time Microns Recovery, % Organics, % E. Pebble 16 2.9 74.0 35.5 Mill hrs.

4. Attrition 7 min. - 69.4 38.1 Mill The results of Table 2 show that sandmilling of oil shale provided a flotation product of slightly lower recovery but at significantly improved grade of organics at a drastically reduced milling time which demonstrates that in accordance with the process of the present invention, kerogen-containing oil shale can be satisfactorily and efficiently beneficiated by froth flotation methods if the organics values are liberated by attrition milling prior to flotation.

Although the present invention has been described with reference to certain preferred embodiments, it is apparent that modifications or changes may be made therein by those skilled in this art. For example, instead of 3 mm glass beads being used as the grinding media in the sandmilling step, other grinding media such as titania or alumina may be used and a vibratory attrition mill may be substituted for the sandmill. Instead of MIBC and No. 2 fuel oil, other known frothers and collectors may be used in the present process to provide enhanced results. Also, as has been mentioned, depressants, flocculants, surfactants and the like may be added prior to flotation to improve the results. All such obvious modifications may be made herein by those skilled in this art without departing from the scope and spirit of the present invention, as defined by the appended claims.

Claims (9)

1. In a froth flotation process for beneficiating fossilized organic mineral values from an ore containing said mineral values and associated inorganic impurities, said process comprising: slurrying particles of said ore in an aciueous medium; conditioning said slurry with effective amounts of a frothing agent and a collector, resoectively; and collecting the organic mineral values by froth flotation methods,the improvement comprising: treating the ore particles in said slurry by attrition milling for a time sufficient to provide a slurry of liberation-sized ore particles prior to flotation.

2. A process as defined in Claim 1, wherein said slurry is attrition milled for a time sufficient to provide a slurry of ore particles having an average particle size of from about 1 to about 10 microns.

3. A process as defined in Claim 1, wherein said slurry of ore particles is first size reduced to provide a slurry feed of ore particles having an average particle size of from about 0.5 to about 5.0 mm prior to attrition milling.

4. A process as defined in Claim 1 wherein said fossilized organic mineral values comprise coal values.

5. A process as defined in Claim 1, wherein said fossilized organic mineral values comprise kerogencontaining oil shale values.

6. A process as defined in Claim 1. wherein said attritioning is performed in a sandmill in the presence of grinding media.

7. A process as defined in Claim 5 wherein said grinding media are selected from the group comprising glass beads, polymer beads, metal beads, ceramic beads or particles, graded sand particles, and mixtures of any of the foregoing.

8. A process as defined in Claim 1 wherein said attritioning is performed by sandmilling by autogenous grinding.

9. A process for beneficiating coal values and or kerogen-containing oil shale values from an ore containing said values and associated inorganic impurities, said process comprising: (a) providing a slurry of finely divided particles of said ore in an aqueous medium; (b) treating the particles in said slurry by attrition milling for a time sufficient to provide an aqueous slurry of attrition milled, liberation-sized coal and or kerogen-containing oil shale particles having an average particle size of from about 1 to about 10 microns; (c) conditioning said attrition milled slurry with effective amounts of a frothing agent and a collector, respectively; and (d) thereafter, collecting the coal and!or kerogen-containing oil shale values by froth flotation procedures.