US3021282A - Heavy medium for gravity separations - Google Patents

Description

United States Patent 3,021,282 HEAVY MEDIUM FOR GRAVITY SEPARATIONS Everett L. Joppa, Duluth, Minn., assignor to P-M Associates, Cleveland, Ohio, a partnership No Drawing. Filed Sept. 4, 1958, Ser. No. 759,049 2 Claims. (Cl. 25260) This invention relates to the benefication art and more particularly to improvements in processes of concentrating or up-grading mineral values by procedures involving use of heavy media, e.g. the float-sink process of separating coal from slate, the hydrocyclone separation of iron ore from gangue material, and the like. The invention is concerned with the provision of a novel heavy medium for use in such gravity separation procedures, of a solid component of such heavy medium, and of a process of preparing such solid component.

As solid components of heavy media there have been used a variety of finely divided mineral solids. From the standpoint of recovering the solid component of the heavy medium from the float-sink products (or, from the underflow and overflow products of the hydrocyclone separation), it is particularly advantageous if said solid component is magnetic. Thus, it heretofore has been proposed to use as the solid component or medium of the heavy medium the product obtained by crushing and grinding magnetite and screening out the particles larger than some arbitrary upper limit of size. The heavy medium wherein such sized magnetite crystals compose the solid constituent is operable, but its use is circumscribed by the limiting factor as to the specific gravity of the resulting heavy medium of usable viscosity characteristics. Ordinary crushed magnetite breaks into irregular, angular shaped particles upon grinding. As is known, the viscosity of a pulp made of a suspension of angular particles in a liquid is greater than that of a similar suspension of rounded particles (U.S. Bureau of Mines, Report of Investigation 3469-R, DeVaney and Shelton). Since a goal in developing heavy media is to provide a pulp which is as heavy as possible while having an acceptably low viscosity, the relatively high viscosity of a pulp of angular magnetite particles materially limits" use of the same.

For improving the specific gravity characteristic of a heavy medium from ground magnetite without unduly increasing its viscosity it has been proposed to admix with the magnetite a greater or lesser amount of finely divided ferrosilicon. However, this latter is several times as expensive as is natural magnetite, and such added expense rules out the use of such mixtures in concentrating procedures applied to very low-cost materials.

It has been found that when powdered coal is burned, in a modern power plant, the iron initially present in the coal is converted to a magnetic mineral which is largely magnetite but contains some iron silicates. The particles of the fly ash so formed are spherical in shape due to the fusion of the coal ash while in a state of suspension. The magnetite content of these fly ash particles is characterized by being extremely fine grained, many of the crystals of which are sub-micron in size, the crystals being encased in and bound together with a slag bond into spheres which strongly resist abrasion and tend to remain spherical in use.

From such fly ash (i.e., the finely divided solid residue from the combustion of powdered coal) there may be recovered, in the state formed, a distinctly magnetic fraction, composed of particles all of which are finer in size than some predetermined maximum and practically all of which are true spheres, which fraction contains more than 53% by weight of iron mostly as magnetite and less than 16% silica, which magnetic fraction can be suspended in water or other suitable aqueous liquid to provide an excellent heavy medium for use in the cyclone separation of iron ore from gangue or in the float-sink process of up-grading coal or similar gravity separation applications. The spherical shape of the magnetic fly ash particles makes this concentrate a particularly valuable material for the purpose stated, because of the materially lower viscosity of their suspension as contrasted with the viscosity of a similar suspension of irregular, angular shaped particles of ground magnetite. Moreover, their extraordinary resistance to abrasion is in contrast to disintegrated natural magnetite, which latter is only a medium hard mineral and which, because of a normally coarse crystalline nature, tends to break down (in use) rather easily, giving undesirable magnetite slimes. These slimes inherently increase the viscosity of the system and tend to increase the media loss which results from the circumstance that it is difiicult for magnetic separators to recover efficiently very fine material (5 microns and finer). Because of the exceedingly extensive surface present in the slime particles the latter tend readily to oxidize to highernon-magneticoxides such as hematite and, hence, are lost in an ensuing magnetic recovery step.

The heavy medium component of the present invention has amongst others, the following advantageous characteristic: From the standpoint of economics, an outstanding characteristic is cheapness, being recovered from a waste product of steam power plants by a simple and inexpensive method which avoids all crushing and grinding. Secondly, the truly spherical form of practically all of the particles constituting the magnetic fractionruakes it possibleas compared with ground natural magnetite-to produce a higher specific gravity heavy medium at the same viscosity (or, in the obverse, to produce a lower viscosity heavy medium at the same specific gravity). The magnetic fraction is more nearly recoverable (from the separation products) than is ground natural magnetite.

It was discovered that grinding this fly ash'either before or after magnetic separation, destroys its usefulness. This is in direct contrast to what might normally be expected. If the material is ground prior to magnetic separation, the desirable spherical nature of the material is destroyed and surprisingly the resultant concentrate contains less iron and more silica which in itself is an undesirable attribute. This comes about because the lower iron content spheres, upon grinding and subjecting to magnetic concentration, yield a percentage of a very low grade iron concentrate that dilutes the remainder of the concentrate. With grinding before concentration, the iron unit recovery is higher, but the product then is inferior both as to grade and as to particle shape. If grinding is pract ced before concentration, the particle shape is then no better than a nautral ground magnetite. If grinding is practiced after the initial magnetic concentration step, the desirable particle shape is likewise destroyed.

The magnetic susceptibility of the magnetic mineral recovered from fly ash is in the same order as that secured from natural magnetite ores. Pure magnetite has a relative attractability of approximately 40.00 using as a scale the measurements proposed by Mr. E. W. Davis in the Minnesota School of Mines Bulletin No. 7. This scale is based on the assumption that pure iron has an attractability of 100. Most magnetites available for commercial use in the heavy media process, because of attached gangue, have an attractability of approximately 35. In separating magnetite from fly ash we purposely use magnetic separators which have a field strength equal to 400 gauss 2 inches from the drum, as contrasted to separators used for recovering magnetite from the medium which have a strength of approximately 600 gauss at the same distance from the drum. This is done so as to discard any of the very lean spherical magnetic particles which would tend to lower the grade of the magnetite recovered. It also insures that only a minor amount of such magnetite will be lost in the separators used to clean up the media in the sink-float or cyclone processes. Tests on various samples of fly ash magnetite indicate that the attractability is in the range of from 30' to 36 on the scale just mentioned. For comparative purposes it should be mentioned that commercial separators with a strength of 700 gauss 2 inches from the pole will recover materials with a magnetic attractability as low as 10.

For producing the solid component of a heavy medium in accordance with the present invention, the fly ash iswithout being crushed, ground or otherwise broken uppreliminarily screened at say, 8 mesh and the oversize is discarded. The minus 8 mesh portion is then subjected to a magnetic separation procedure and the non-magnetic portion is discarded. Finally, from the magnetic portion there is separated a fraction all of the particles of which are finer than a predetermined maximum, and the particles larger than said maximum are discarded. The so-obtained fraction is suspended in aqueous liquid to provide the novel improved heavy medium of the present invention.

By the use of the expression predetermined maximum reference is here made to the fact that the maximum size of particle selected is related to the particular beneficiation application under consideration. Where, as in the hydrocyclone beneficiation of low-grade iron ore maat'efials, a relatively coarse medium is of advantage, the fpredeter'mined maximum size of medium may be as coarse as 35 'mesh. Atthe other extreme, where-as in the tip-grading of coal-float-sink separators of the non-setting type are sometimes used and a relatively fine rnedium is 'desired, the predetermined maximum size of particle maybe as fine as 100 mesh. In either event, the'frac'tion is composed of spherical particles of a variety of sizes ranging from the aforesaid predetermined maxiinner to particles finer than 325 mesh, the greater part of the total being minus 325 mesh.

Three fly ashsamples, from as many sources, were examined. The following structure analysis data were secured:

' STRUCTURE ANALYSIS Fly ash samples, as received Sample ".A] Sample 13 Sample Size Percent Accm. Percent Accm. Percent Acorn.

Wt. Wt. Wt.

Total 100. 00 100. 00 100. O0 100. O0 100. 00 100. 00

' SampleA 18.57 Sample 26.79 'Sample"C- 23.66

4 with varying amounts of silica, alumina, lime, magnesia, manganese, phosphorus and sulphur.

The magnetic mineral recovered from fly ash has a truly spherical shape.

Chemical examination of the samples showed that the finer the particle the higher was the iron content and the lower was the silica content. The following data, pertinent to sample B, are generally illustrative of all three samples.

SCREEN ANALYSIS OF SAMPLE 13" 8 mesh magnetic separator concentrates Assay, percent Size Percent Iron Silica 3. 61 l 25. 31 2. 25. 47 2. 47 25. 07 2. 08 25. 1. 70 26. 44 1. 41 28. 53 42. 52 l. 28 35. 30 36. 41 l. 16 46. 91 22. 93 1. 50 49. 81 20. 44 3. 51 53. 68 16. 36 4. 88 57. O6 12. 90 24. 36 59. 96 10. 07 49. 24 63. 83 7.00

Total 100. 00 56. (19

Composzte results 48 Mesh FractionunQ 85.93% Wt.-." 61.04% Iron, 9.47% Silica. 65 Mesh Fractiou 84.65% Wt 61.43% I'ron, 9.07% Silica. -100 Mesh Fraction 81.99% Wt 61.84% Iron, 8.66% Silica.

The aforesaid magnetic con'centratejof fly ash com-- pared with ground natural magnitite, as to structure, grade and specific gravity, as follows:

FLY ASH MEDIA vs. GROUND MAGNETITE STRUCTURE, GRADE AND SPECIFIC GRAVITY Ground Size Fly Ash Magne tlte 65 1.49 -e 20M 1. 35 100M 1. 75 2. 43 M 4.08 4. 67 znnM 5. 68 7.48 325M 28. 35 18. 30 --325M 57. 30 67. 12

Total 100. 00 100. 0D

61. 04 64. 09 9. 47 9. l8 Specific Gravity 4. 39 4. 53

The invention will now be described in'greater particularity with reference to the following illustrative specific examples.

EXAMPLE 1 In this example are shown the results of making a floatsink separation on minus 4 inch plus one-fourth inch bituminous coal containing shaley impurities.

In one case typical ground magnetite containing approximately 2.4% by weight coarser than 100 mesh and 67% finer than 325 mesh was used as medium. In the comparative tests, spherical magnetite from fly ash was made by taking furnace ash, rejecting the +8 mesh material and magnetically separating the undersize and again rejecting the portion of the concentrates coarser than 48 mesh. In the two comparative tests the specific gravity of the medium was held at the same point, namely, 1.45. Because of the fact that 'thefflyash magnetite was spherical it had a considerably lower vis cosity than had the identical suspension made up of ground natural magnetite. This lower viscosity permitted a more clearcut separation between coal and shaley impurities to be made. The magnetite from the fiy ash, because of its shape, could be more completely removed and recovered in the screening and magnetic concentration stages of the recovery processes than was the normal type of ground magnetite. These results are shown in the accompanying table, where it may be seen that the magnetite consumption dropped from 1.05# per ton of feed to .35# per ton of feed. It may also be seen that the use of this special magnetite resulted in a sharper separation of the coal from the waste, and also made it In certain geographical areas it is sometimes diflicult to find gravel, that is sufficiently free from undesirable material, such as shale, that can be used as aggregate to give a concrete of high strength. In recent years it has become rather commonplace to treat such gravel with a heavy media process. For such an operation a gravity in the order of 2.45 is necessary to separate the softer and lower density shales and like materials from the denser constituents of gravel. Most ground magnetites have a density of approximately 4.8. With such magnetites it is possible to secure a suspension close to the desired density if the media is perfectly clean and there is no slime contamination. Under normal conditions, however, the natural magnetite suspensions are viscous and it is difiicult to maintain a sufiiciently high gravity to make an effective separation. Many operators have therefore found it expedient, in order to secure a sufficiently high gravity together with a reasonable viscosity, to use a mixture of ferrosilicon and magnetite. This results in considerable extra expense since ferrosilicon normally costs five times as much as magnetite. It has been found that the magnetite produced from fly ash because of its spherical nature will have, at the same densities, a considerably lower viscosity than if the regular ground natural type of magnetite is used. It is therefore possible to use this fly ash type of magnetite alone without the necessity of ferrosilicon additions for this purpose.

EXAMPLE 2 This example summarizes test work on a sample of gravel requiring upgrading. In one case a mixture of ground magnetite and ferrosilicon was used and in the other the preferred fly ash magnetite was employed. The pulps were held at the same specific gravity. With the preferred type of magnetite, the amount of misplaced material in the aggregate of sink products was only 1%. Where a mixture of ground magnetite and ferrosilicon was used, the amount of such misplaced material was 1.5%. As in Example 1, the media loss using the preferred material was considerably less than for the mixture of natural magnetite and ferrosilicon. This substitution of media represented a saving of 2 per ton of crude material which, with a low value product such as gravel, amounted to a very substantial saving.

The comparative results obtained are shown in Table 2 following:

6 TABLE 2 Float-sink separation of gravel for concrete aggregate The use of hydrocyclone has come into general use in concentrating fine sizes of iron ores as well as some other minerals. On the Mesabi Range in Minnesota it is widely used for treating the washed portion of the ore. In such operations the media is usually made up of ground magnetite and to a lesser degree, magnetic sinter fines. The preferred size of such matrial is usually -48 mesh. Because part of the separating effect is due to centrifugal force as well as to the density of the medium, it is not necessary to employ as dense a medium as is utilized in the sink-float separation of such ores. Normally a pulp is maintained that has a specific gravity of from 2.30 to 2.40. Example 3 gives the results of a cyclone separation test made on an ore of the type described. Examples are shown of the results using ground magnetite as compared to the magnetite made from fly ash. Because of the lower viscosity possible with a preferred variety of magnetite it was possible to maintain a specific gravity of media of 2.45 compared to the usual 2.38 and still maintain a workable viscosity and this resulted in producing a superior grade of concentrate with a lower media consumption per ton of material. In this particular case the reduction in the amount of magnetite consumed amounted to 2.5# per ton of feed which represented a saving of approximately 4. On the basis of the concentrate produced, the saving amounted to 9.1. It will also be noted that, due to the higher gravity used in the cyclone operation, the iron content of the concentrate was increased from 54.46% to 56.72%. There was a corresponding reduction in the silica content.

The comparative results obtained are set forth in Table 3 following:

TABLE 3 Cyclone separation of Mt" hematite ore fines 1. A heavy medium for use in the gravity separation of a mixture of solid mineral particles of different specific gravities, said heavy medium consisting essentially of an aqueous liquid suspension of a magnetic fraction of fly ash composed of generally spherical particles consisting essentially of magnetite and of varying sizes all of which are finer than 35 mesh and the greater part of which are finer than minus 325 mesh particles, the particles of said magnetic fraction assaying more than 53% by weight of iron mostly as magnetite and less than 16% by weight 7 of silica, said heavy medium having a specific gravity within the range 1.45 and 2.45.

2. A heavymedium for use in the gravity separation of a mixture of solid mineral particles of different specific gravities, said heavy medium consisting essentially of an aqueous liquid suspension of a magnetic fraction of fly a'sh composed of generally spherical particles consisting essentially of magnetite and of varying sizes all of which are finer than 35 mesh and the greater part of which are finer than minus 325 mesh particles, the particles of said magnetic fraction assaying more than 60% by weight 'of iron mostly as magnetite and less than 10% by Weight of silica, the particles composing said fraction having a 8 specific gravity'inexc'ess of 4.0 and said heavy medium having a specific gravity within the range 1.45 and 2.45.

References Cited in the file of this patent UNITED STATES PATENTS 1,958,351 Shingshang May '8, 1934 2,122,236 Nichols etal. June 28, 1938 2,430,186 Rakowsky Nov. 4, 1947, 2,686,593 Vogel et a1 Aug. 17, 1954 2,774,734 Rodis et al.v Dec. 18, 1956 2,878,518 Klee Mar. 24, 1959

Claims (1)

1. A HEAVY MEDIUM FOR USE IN THE GRAVITY SEPARATION OFA MIXTURE OF SOLID MINERAL PARTICLES OF DIFFERENT SPECIFIC GRAVITIES, SAID HEAVY MEDIUM CONSISTING ESSENTIALLY OF AN AQUEOUS LIQUID SUSPENSION OF A MAGNETIC FRACTION OF FLY ASH COMPOSED OF GENERALLY SPHERICAL PARTICLES CONSISTING ESSENTIALLY OF MAGNETITE AND OF VARYING SIZES ALL OF WHICH ARE FINER THAN 35 MESH AND THE GREATER PART OF WHICH ARE FINER THAN MINUS 325 MESH PARTICLES, THE PARTICLES OF SAID MAGNETIC FRACTION ASSAYING MORE THAN 53% BY WEIGHT OF IRON MOSTLY AS MAGNETITE AND LESS THAN 16% BY WEIGHT OF OF SILICA, SAID HEAVY MEDIUM HAVINGA SPECIFIC GRAVITY WITHIN THE RANGE 1.45 AND 2.45.