CA1175639A - Separation of high grade magnetite from fly ash - Google Patents

Abstract

ABSTRACT
A unique, high-grade magnetite obtained from fly ash, the use thereof in the cleaning of coal, and low iron content fly ash.

Description

3~

S~PAR~TION OF HIGH GRADE
MAGNETITE FROM FLY ASH

Electric utilities as of 1978 consumed nearly 470 million tons of coal annually in the United States. Due to ever increasing costs of petroleurn-based fossil fuels and a national energy policy of reduc-ing dependence on foreign-source fuel such as oil ~y shifting to coal, electric u-tilities are now projected to use nearly 800 million tons of coal annually by 1985. Fly ash, the predominant residue of 10 coal burning, has in the past presented disposal problems to users of significant amounts of coal. Despite the natio~,?al focus on re-source recovery and recycling during the recent past ~ecades, and the doubling of the percentage of fly ash utilized over the period 1966 to 1978, the year 1978 saw the collection of over 48 million :L5 tons of fly ash by electric utilities alone and the utilization of only 8 million tons of that total. An estimated 68 million tons of fly ash are annually produced in the United Sta-tes and the significant and perhaps e~ren dramatic anticipated shift from petroleum to coal in fossil fuel generating stations can be reasonably expected to greatly 20 increase the amount of fly ash collected in the future.
Although to date many coal~fired generating stations have been located near sources of coal where ash disposal problems may be presumed to ~e minimal, as oil-fired uni~s far removed from coal fields convert to coal under the contemporary pressures of econo-25 mics and national policies, ash disposal can be expected to developinto an ever-increasing problem which, when coupled with increas-ingly stringent federal, state and local regulation of landfills, water quality and waste disposal ~enerally, will present significant chal-lenge and expense to such large scale coal users. Of the 8 million 30 tons of fly ash utilized in 197~, almost one-third of that was utilized from disposal sites, i . e ., after the producers of the fly ash had already incurred disposal costs.
Of the approximately 8 million tons of fly ash utilized in 1978, about two-thirds of it was used commercially in such applications as 35 concrete products, cement, fill and the like.

Through the process of the invention abou-t fifteen weight percent of raw fly ash can be magnetically separa-ted out as hi~h grade magnetite.
The fraction comprising the remaining 85 percent of raw fly 5ash con-tains less than about 40 percent, typically from about 15 -to 30 percent, of its original iron content. Removal of -the 15 percent of the fly ash making up the magnetic fraction would significan tly reduce d;sposal-related -transportation costs and extend the life of fly ash disposal sites. The non-magnetic fly ash fraction provided 10by the invention has a specific gravity of less than about 2 . 2, usua]ly from about 1. 9 to about 2 . 2 . Moreover, the low iron con-tent fly ash residue of the process provides a significantly altered product when considered in the light of both presently known comrnercial utilizations of fly ash and utilizations contemplated for 15the future. For example:
(1) Embankment and structura_ - The non-magnetic residue, typically having a specific gravity of 2.1, is somewhat lighter per unit volume than the original fly ash, which has a typical specific gravity of 2 . 5 . This property is important in 20the use of fly ash for embankments or in structural fill applica-tions .

(2) Treatment of polluted waters - In some applications, a low iron content is desirable, as well as a low speci~ic gravi-ty.
25(3) Soil n_tralization and fertilizers - In these cases, a high Ph is a desirable fea-ture, and low iron contents for at least some fly ash materials will give this desired property.
(4) Mine rec]amation - For use in mine reclamation, a fly ash with a lowered iron content will result in the formation of 30lower acid content mine water runoff, and is thus to be desired .
(5) Concrete_ocks - A low iron content in this appli-cation will result in a reduction of b]ock staining upon stand-ing to weather.
35(6) C,em_nt manufacture - The applicability of the non-magnetic fly ash fraction for this particular application may be enhanced by its lower iron content.

-3~

Known prior art processes relating to the separation of fly ash into magnetic and non-magnetic fractions have generally employed the magnetic constituents in minimal -technology areas. See, for example, U.S. Patents Nos. 1,512,8~0 to Ulrich et al. (building sand or stone) and ~,057, 512 to Vadovic et al. (landfill and blast furnace feed). Magnetic separation has also been employed in the concentration of iron ore (see, for example, U. S . Patents Nos .
2,692,050 to Nelson; 2,990,12~1 to Cavanagh et al.; and 3,198,~22 to Herzog et al ) and -the cleansing and concentration of asbestos (U . S . Patents Nos. 3,42'1,307 to Shiuh and 3,493,108 to Martinez, respectively) .
Coupled with the availability of fly ash as a resource for magnetite and the resultant savings in disposal cos-ts is the need for magnetite in the "hard rock" cleaning industry and an increas-ing need for coal cleaning, which process represents a second major use for magnetite.
As coal use expands, coal quality is steadily decreasing as prime coal seams are depleted and producers turn to the use of mechanized and continuous-mining methods. When ground, coal may be separated from much of the rock and other ash-forming consti-tuents })y a flotation-type separation process utilizing magnetite in admixture with water. See, for e~ample, IJnited States Patents Nos . 3,463,310 to Ergun et al .; 3,583,56Q to Cline; 3,737,032 to Burkitt; 3,79~,162 to Miller et al.; 4,0~8,228 to ~erris et al.; and

4,~40,628 to Horsfall.
~oal cleaning can reduce up -to 65 percent of ash, resulting in improved boiler availability and reliability, especially with older boilers. Cleaning eliminates waste produc-t that may account for as much as 15 to 20 percent of the mass of raw coal, thereby reducing shipping expenses. Also, coal cleaning offers the possibility of eliminating some of coal's inorganic sulfur content prior to combus-tion, thereby reducing the load on flue gas desulfurization equip-ment and therefore reducing costs associated therewith, such as for sludge disposal and limestone. As much as 30 to 50 percent of -the total sulfur content of coal may be subject to removal by coal clean-ing, a consideration highly relevant to the reduction of acid rain.

~5~3~
-3a-Thus the present invention provides in one embodiment a high purity magnetite derived from fly ash which is the pro-duct of coal combustion obtained by:
(a) subjecting a slurry derived from fly ash to a first wet magnetic separation;
(b) screening the magnetic fraclion from said first wet magnetic separation;
(c) subjectin~ ~e ~versized par~icles to grinding;
(d) screening the products from said ~rinding step;
(e) subjecting the passed material from the screenin~
steps (b) and (d) to a final wet magnetic separation; and (f ) separating a high purity magnetite from said final wet separation.
In another aspect the in~ention provides magnetite derived from fly ash obtained as a product of coa7 combustion which comprises an admixture of spherical particles and broken spherical particles, the broken spherical particles being obtained by grinding spherica7 particles having a size greater than 325 mesh; said ma~netite having a percent magnetics of at least about 96~ as measured by Davis Tube and a speciic gravity of from about 4.1 to about 4.5 and consisting essen-tially of particl~s less than 325 mesh.
In still another embodiment the invention provides a process for recovering magnetite from fly ash obtained as a product from coal combustion, said process including the ~teps of:
(a) subjecting a slurry derived from fly ash to a first wet magnetic separation;
(b) screening the magnetic fraction from said first wet magne$ic separation;
(c) subjecting the oversized particles to grinding;
(d) screening the products from said grinding step;
(e) subjecting the passed material from the screening steps (b) and (d) to a final wet magnetic separation; and (f ) separating a high purity magneti-te from said final wet separation.

, 4 ~ 3~

Acid rain has been a recognized problem for some time. In addition to damaging soil, priceless and irreplaceable monuments and stone buildings, and reducing visibility, the phenomenon of acid rain kills fish in freshwater lakes and damages plant life. Acid rain has already eliminated fish from over 100 of New York State's Adirondack Mountain lakes and is rapidly killing lakes in nearby eastern Canada . Both United S tates and Canadian government officials have recently announced intensive efforts to attack the acid rain problem. The source of the acid precipitation is coal burning:
sulfur and nitrogen oxides emitted from smokestacks are swept aloft, combine with atmospheric water vapor to form dilute sulfuric and nitric acids, and the corrosive water vapor condenses and precipitates. The problem can only be expected to intensify as more coal is burneA and as coal burners seek to use higher sulfur content coal.
Magnetite is commonly used in coal cleaning ins l:allations to form the heavy medium for beneficiation, whether the separation process employed is static or centrifugal. The preference for magnetite as the heavy consti-tuent of the separation medium arisec primarily from the ability to easily recover the magnetite by means of magnets and reuse it.
Heretofore virtually all magnetite used in coal cleaning~ has been natural magnetite or mill scale, the magnetic oxide scale result-ing from hot metal forming processes. Commercial grades of natural magnetite exhibit specific gravities in the range of 3.9 to 5.2, with 4 . 2 to 5 . 0 being the most common range . Typically the better grades of commercial magne-tite comprise about 95 percent magnetics.
The pre~erred particle size consist for magnetite used in coal clean-ing operations is -325 mesh.
According to the present invention, magne-tite having a specific gravity as high as 4.5 and comprising as much as 98 percent mag-netics may be produced from fly ash. A preferred process com-prises both dry and wet magnetic separation and grinding.
IN THE FIGURES:
FIG. 1 is a scanning electron microscope (SEM) photomicro-graph (approximately 1000X) of natural magnetite;

-5--FIG. 2 is an SEM photomicrograph (2000X) of raw fly ash;
FIG. 3 is an SEM photomicrograph (2000X) of ground fly ash;
FIG. ~ is an SEM photomicrograph (2000X) of magnetite sepa-rated from fly ash;
E IG . 5 is an SEM photomicrograph (2000X) of fly-ash derived magnetite ground to -325 mesh.
As indicate~ by a comparison of rIGS. 1 and 2, natural magne-tite comprises rouqhly bar-shaped particles having sharp angular configurations whereas fly ash particles have a spherical shape. As sugges-ted by a comparison of FIGS. 2 and 3, -the spherical particles of raw fly ash comprise hollow spheres containing smaller spheres, with the wall making up the large broken sphere of ~IG. 3 exhibit-ing spherical cavities (see generally Fisher et al., Fly Ash Collected from Electrostatic Precipi-tators, 192 Science 553-555 (May 1976)).
Magneti-te recovered from fly ash is comprised almost entirely of spherical particles as indicated by FIG. 4 and fly-ash derived magnetite -that has been ground to -325 mesh contains a large proportion of round particles, as shown in E?IG. 5.
The separation of magnetite from fly ash according to the invention utilizes wet drum magnetic separation and preferably includes both dry and wet magnetic drum separation, with the dry separation step(s) occurring prior to wet separation. Dry separa-tors typically maximize the quantity of magnetic material separated out of the raw fly ash feed while the wet separation enhances the quality of the recovered magnetite, apparently by aiding in the elimination of fine clay particles from the magnetite.
In the absence of any grinding of the material, magnetite recovered fr~m fly ash in the manner here described typica~ly comprises in excess of about 90 percent magnetics, most preferably in excess of 96 percent magnetics; has a specific gravity of at least about 3.9; and has a size distribution of 60-70 percent -325 mesh.
Screening of fly ash magnetite at 325 mesh typically yields an over~;ze material lower in specific gravity and significantly lower in percent magnetics when compared to the magnetite passing the screen. If, however, the oversize fraction is ground to pass 325 mesh and again subjected to magnetic separation, both the specific gra~ity and percent magnetics of the magnetite so recovered are

-6~ 63~

markedly higher, with the specific gravity rising to about 4.1 to about 4 . 5 and the percent magnetics to as high as about 9~ per-cen~. The difierences in the recovered ground material are be-lieved attributable to the release of a more dense and hicJhly magne-tic material by the fracturing of larger spheres and ~he elimination of relatively lighter and non-magne tic materials comprising the shells of the larger spheres. It is therefore preferred, when a higher density magne-ti-te product is sought, to grind at least the magnetic fraction of the fly ash to -325 mesh prior to -the final ~vet magnetic separation.
The higher density magnetite has economic signiiicance when used in coal separation plants utilizing heavy media having specific gravities ranging between 1. 3 and 1. 5 because a smaller volume of the higher specific gravity magne-tite is required in the magnetite-water slurry to form the required medium. Where specific gravities greater -than 1. 5 are used, the lighter material must be used in greater volume Lo achieve a desired specific gravity in the heavy medium, and the crowding attendant a large particle population in a given water volume tends to increase fluid viscosity which, in turn, results in impurities not separating from the coal as rapidly as one would like. Thus separation efficiency may be somewhat impaired.
It should be recognized, however, that less dense magnetité, in requiring a larger particle population to sustain any given heavy medium specific gravi-ty, tends to result in a more uniform heavy medium that conforms more closely to the behavior of a true liquid than a heavy medium employing a higher specific gravity magnetite.
A magnetite product according to the invention contains a high proportion of round particles, the presence of which in~luences the performance characteristics of the magnetite. It is known, for example, as noted above, that excessive viscosity of the dense medium is detrimental to the separation of coal, particularly in static vessels. Round magnetic particles reduce viscosi-ty by de-creasing the resistance of the par-ticles to movement both past each other and through the liquid component of the medium.
The higher percent magnetics offered by magne-tite according to the invention results in a reduction of slimes (non-magnetics), and thereby lessens maintenance, in coal-cleaning systems utilizing

- 7-same and reduces the money paid for non-functioning maeerial The larger size consist of the fly-ash derived magnetite, coupled with the improved magnetic ~luality, improves the material handling characteristics of such systems.
In a preferred manner of use of the present invention, large coal users, s-~lch as elec-tric utilities, located in the vicinity of raw cnal supplies or o-therwise havillg incentives for installing coal cleaning facilities (e.g., desulfuriza-tion), would provide raw f~y ash input for the magnetite recovery process, thereby reducing fly ash disposal costs and enhancing ~he utility of the non-magnetic fly ash fraction; a portion of the magnetite so produced would be utilized on site in coal-cleaning processes, thereby obviating the need to purchase magnetite for same; and the remainder of the magnetite produced wou]d be marketed.
Processes according to -the invention are detailed in the following examples:
Example I
Raw fly ash (approximately 125 kilograms3 produced in a pul-verized coal-fired utility boiler is fed to a high speed, permanent magnet dry drum separator at 500 FPM, 4 TPH/FT and 1, 000 gauss field int:ensity and magnetic, middling and non-magnetic fractions collec-ted. The non-magnetic fraction is sent to a 15-inch diameter double-roll criss-cross dry drum magnetic separator, with the non-magnetic fraction yielded in the Eirst pass re-passed through the criss-cross dry drum, the feed rates and field strengths in both passes being approximately 1,500 LBS/HR/~T and 1,000 gauss, respectively. All magnetic -frac-tions and the middling fraction from the first separator are slurried to about 25 percent solids in water and fed to a wet drum ma~netic separator at 3 GPM and a field strength of 1, 000 gauss . Non-magnetic and magnetic fractions are obtained, filtered and dried. The magnetic product Irom the wet drum is then repassed in the wet drum to yield a concentrate of 96.6 percent magnetics, the percent magnetics being determined by Davis Tube, and a specific gravity of 3.9.
Example II
16.1 kilograms of fly ash produced in a pulverized coal-fired electric utility boiler, containing 10 . 5 percent ma~ne-tics by Davis

-8~

Tube, are passed through a permanent magnet dry drum separator at approximately 1,000 gauss field strength. The non-magnetic fraction, representing approximately 71 percent of the feed, is passed throuyh a second dry drum separator at approximately the 5 same field s trength . The magnetic fractions of both drums, rep-resenting approximately 32 percent of the feed, are then mixed with water to form a slurry of approximately 20 percent solids by weight and passed through a wet drum permanent magnet separa-tor at about 3 GPM . Field intensi-ty for the wet drum is 1, 000 gauss .
10 The magnetic product is collected, weighed and its magnetics con-tent determined using Davis Tube to show a magnetic product of 96 .1 percent magnetics, representing 8 . 4 percent by weight of the feed and an 84 . 6 percent total recovery of available magnetics .
The specific gravity of the magnetic product is 3.9.
_xample III
Fly ash ~approximately 2,727 kilograms) produced in a pulver-ized coal-fired utility boiler is mixed with water in a slurry mixing tank. The solids are adjusted to approximately 25% by weight and fed to a 1,000 gauss wet drum separator. The non-magnetic frac-20 tion has a specific gravity of 2.1 and the magnetic fraction isre-passed to obtain approximately 300 kilograms of magnetic product at 91 percent magnetics as measured by the Davis Tube and having a specific gravity of 3 . 9 . The magnetic product is ~:ested in an eight-inch, heavy medium cyclone coal-c~eaning circuit and the 25 quality of coal product obtained is comparable to that attainable using commercially available natural magnetite.
Exam~le IV
A 4.5 kilogram sample of the 300 kilogram magnetic product of Example III is screened at 325 mesh. The -325 mesh portion con-30 tains 95 percent magnetics by the Davis Tube and the o~ersizematerial, 88 percent magnetics. The oversize material, representing abou~ 30 percent of the starting sample, is ground in a jar mill to pass 325 mesh. After final separation in a wet magnetic separator, the magnetics content of the magnetic portion of -the ground material 35 increases to 96 percent and the material has a specific gravity of 4 . 2 . Fly ash magnetite, with a size consist essentially -325 mesh, 96 percent maynetics and a specific gravity of 9 . 2, is mixed with

9-water in the head tank of a heavy medium cyclone coal beneficiation plant to produce a slurry density appropriate for the separation of pyrites and ash-forming impurities from a bituminous coal. ~fter screening the coal feed using wet sizing screens and sieve bends, 5 the resulting 1~41~ X 28 mesh size consist is mixed wi-th the magnetite slurry in the head tank. The mixture thereupon is fed to the inlet of a 14-inch cyclone, where the combination of centrifuyal force and gravity in the presence of the magnetite/H2O slurry serve to sepa-rate coal from non-coal constituents. The overflow mixture of coal,

10 magnetite and water is passed over a sieve bend and then a vibrat-ing wet screen. The washed coal is essentially magnetite free and passes to other unit operations in the plant. The dilute medium, predominantly magnetite and rinse water but with some nonmagnetic particles, is passed to the final wet drum separation section of the 15 fly ash magneti-te wet magnetic separator. The wet magnetic sepa-rator recovers and concentrates the magnetite which is sent again, along with fresh makeup from the same separator, -to the heavy medium cyclone head tank. The nonmagnetic particles which other-wise would accumulate in the heavy medium cyclone circuit are thus 20 removed from the system. Similarly, the underflow from the heavy mediurn cyclone, containing magnetite, water, and rock and mineral matter is classified using a sieve bend and wet vibrating screen with the oversize material, basically free of magnetite, reporting to refuse and the dilute medium reporting to the final fly ash magnetic 5 separator for the same purpose as previously described.
Example V
A 500 gram sample of the magnetic product produced in Exam-ple I is screened at 325 mesh. Approximately 36 . 5 percent of the magnetic product is oversize material. The oversize material is 30 ground in a laboratory jar mill to pass 325 mesh, slurried with water and finally passed through a magnetic separator. The mag-netic product contains 98 percent magnetics by Davis Tube and has a specific gravity of 4 . 3 . Fly ash magnetite, with a size consist essentially -325 mesh, 98 percent magnetics and a specific gravity 35 of 4 . 3, is mixed with water in the head tank of a heavy medium vessel coal beneficiation p]ant. Coal, wet screened and sized at +~4 inch, is added to and mixed in the head tank and the resulting -10- ~L'7~3~

feedstock introduced to the heavy medium vessel. The amount of mas~netite added to Lhe water -to form the heavy medium is calculated based on the specific gravity of the coal product and is generally chosen such that larye proporLions of the product do not fall within 5 10% of -the specific gravi-ty of the medium. The overflow ma-terial from the bath, consisting mostly of coal, magneti-te and water, is screened and washed to separate coal product for shipment or fur-ther processing. The finely clivided magnetite passes through the screens, is separated thus from the coal, and is sent to the 10 final wet magnetic separator section of the a~ove-described fly ash magneti-te recovery process. The magnetite is separated and COIl-centrated in the magnetic separa-tor and sent again, with fresh makeup, to the heavy medium vessel head tank. Nonmagnetic slimes, otherwise accumulating in the coal-cleaning circuit, are 15 removed. The underflow material from the heavy medium vessel, containing wa-ter, magnetite and refuse materials, undergoes similar processing to recover and concentrate magnetite for reuse and to isolate coal refuse and medium slimes for disposal.

Claims (13)

What is claimed is:

1. A high purity magnetite derived from fly ash which is the product of coal combustion obtained by:
(a) subjecting a slurry derived from fly ash to a first wet magnetic separation;
(b) screening the magnetic fraction from said first wet magnetic separation;
(c) subjecting the oversized particles to grinding;
(d) screening the products from said grinding step;
(e) subjecting the passed material from the screening steps (b) and (d) to a final wet magnetic separation; and (f) separating a high purity magnetite from said final wet separation.

2. Magnetite derived from fly ash obtained as a product of coal combustion which comprises an admixture of spherical particles and broken spherical particles, the broken spherical particles being obtained by grinding spherical particles having a size greater than 325 mesh; said magnetite having a percent magnetics of at least about 96% as measured by Davis Tube and a specific gravity of from about 4.1 to about 4.5 and consisting essentially of particles less than 325 mesh.

3. The magnetite of claim 2 wherein the major portion of the magnetics is in the form of spherical particles.

4. A process for recovering magnetite from fly ash obtained as a product from coal combustion, said process including the steps of:
(a) subjecting a slurry derived from fly ash to a first wet magnetic separation;
(b) screening the magnetic fraction from said first wet magnetic separation;
(c) subjecting the oversized particles to grinding;
(d) screening the products from said grinding step;
(e) subjecting the passed material from the screening steps (h) and (d) to a final wet magnetic separation; and (f) separating a high purity magnetite from said final wet separation.

5. The process of claim 4 wherein the oversized particles from screening step (d) are recycled to the grinding step (c).

6. The process of claim 4 wherein the high purity magnetite from the final wet separation step is dried.

7. The process of claim 4 wherein the passed material fed to the final wet magnetic separation passes a 325 mesh screen.

8. The process of claim 4 wherein the high purity magnetite has magnetics of at least 90% and a specific gravity of at least 3.9.

9. The process of claim 4 wherein the final magnetite has a percent magnetics of at least 96% and a specific gravity of from about 4.1 to about 4.5

10. An integrated process for recovering magnetite from fly ash obtained as a product of coal combustion according to the process of claim 4 and for cleaning coal prior to its combustion, said process including the additional step of utilizing the magnetic product of the final wet magnetic separation in the heavy medium beneficiation of coal.

11. The integrated process according to claim 10 including the additional step of recovering magnetite used in said heavy medium beneficiation in said final wet magnetic separation step.

12. The high purity magnetite of claim 1 wherein the slurry derived from fly ash in step (a) is obtained by first subjecting the fly ash to a dry magnetic separation and thereafter forming a slurry of the magnetic fraction obtained from said dry separation.

13. The process of claim 4 wherein the slurry derived from fly ash in step (a) is obtained by first subjecting the fly ash to a dry magnetic separation and thereafter forming a slurry of the magnetic fraction obtained from said dry separation.