Note: Descriptions are shown in the official language in which they were submitted.
3~
1063-1
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.
--2--
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.
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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 magnetic5 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.
