Manufacture of gypsum board from FGD gypsum

By-product gypsum, which is formed during a flue gas desulfurization process, (FGD gypsum) is carefully formed as an aqueous slurry of crystals having a mean particle diameter of between about 35 microns and 80 microns and filtered, which filter cake is substantially completely dried before calcination, without any substantial grinding action, the crystals are then flash calcined to a hemihydrate, then ground, to reduce consistency, and then formed into an aqueous slurry from which a paper-covered gypsum board is formed of superior core quality.

This invention relates to a process for making improved gypsum board from 
FGD by-product gypsum. 
Presently substantially all domestic gypsum wallboard is manufactured using 
natural gypsum. The use of synthetic gypsum, such as the by-product from 
phosphoric acid production, in manufacturing gypsum wallboard is known, 
and is discussed in prior U.S. patents. 
The by-product of phosphoric acid production has certain disadvantages in 
such use, including, particularly, an excessive amount of radioactivity, 
considering how such a product might be used in substantial quantities to 
form all the walls and the ceilings of a room. 
To avoid this problem, it has now been found how a superior gypsum 
wallboard can be produced using a form of gypsum which is the by-product 
of a flue-gas desulfurization process (hereinafter referred to as FGD). It 
should be stated that radioactivity is not a problem when FGD by-product 
gypsum is used. In addition, the FGD gypsum is generally made at a purity 
of 94-98% purity, whereas natural gypsum deposits are generally less than 
90% purity. 
The FGD by-product gypsum can be carefully formed to produce gypsum 
crystals having a mean particle diameter of between about 35 microns and 
80 microns. This FGD by-product gypsum can be produced as an aqueous 
slurry consisting of relatively pure calcium sulfate dihydrate. If the 
slurry is thickened and filtered on a drum, belt, or centrifuge filter 
system, the free water present can be reduced to 10-15% based on the 
weight of the gypsum. 
In accordance with the invention, the above-described filter cake of FGD 
gypsum crystals is subjected to a flash drying and deagglomeration 
process, to eliminate preferably 100% of the free moisture present, which 
may also unavoidably remove a minimal percentage of the combined moisture 
of the FGD dihydrate gypsum crystals. 
This dried product is subjected to a Calcidyne flash calcination process, 
as disclosed for use with natural gypsum in U.S. Pat. No. 3,956,456, to 
remove 75% of the combined water in the FGD dihydrate gypsum molecules. 
This flash calcined FGD hemihydrate gypsum, or a portion of it, is then 
subjected to a grinding process in a mechanical impact-type mill employing 
a single row of pins on a high speed rotor and a companion row of pins 
formed into the mill housing liner. 
The resultant product has been found to surprisingly function to produce a 
superior gypsum wallboard, when combined with water and formed between two 
paper liners. 
Some of the reasons why this result was unexpected relate to physical 
phenomena that cannot be fully understood. All of the prior commercial use 
of the Calcidyne flash calcination process of U.S. Pat. No. 3,956,456 has 
been with natural gypsum which was ground to a fineness of about 10 to 20 
microns average particle diameter, based on a Leeds-Northrup Microtrac 
analysis. The ability of the Calcidyne flash calcination process to 
produce a high quality calcined gypsum from the much larger particle size 
FGD gypsum was therefore not expected. Since the Calcidyne flash calcining 
process produces uniformly calcined larger particles with the FGD gypsum 
it is now possible to reduce the particle size to the desired size after 
calcination rather than before, and since the calcined hemihydrate 
material can be reduced in particle size easier than the uncalcined 
dihydrate, a very substantial savings in power required for grinding is 
provided. 
Since post-grinding flash calcined natural gypsum normally results in no 
change or an increase in the water demand, it was an unexpected advantage 
to find that post-grinding the FGD flash calcined gypsum lowered the water 
demand, making possible lower drying costs when used for manufacturing 
wallboard. 
It is an object of the present invention to employ a certain advantageous 
by-product gypsum in the production of gypsum wallboard. 
It is a further object to provide a process for employing FGD by-product 
gypsum in the production of gypsum wallboard. 
It is a still further object of the invention to produce a superior gypsum 
wallboard, using FGD by-product gypsum as the starting material.

In accordance with the present invention, gypsum wallboard is produced 
using, as at least a substantial portion of the gypsum starting material, 
a by-product gypsum produced in desulfurization of a flue gas. One such 
desulfurization process is disclosed in U.S. Pat. No. 3,836,630. This 
process, and other flue gas desulfurization processes, can produce 
by-product gypsum slurries having a solids purity of from 88% to 98% 
gypsum, average gypsum particle diameters of from 35 to 80 microns, 
preferably from 60 to 70 microns, and gypsum crystal shapes with an X:Y 
ratio less than 10, which are thus suitable for use in the processes of 
the invention. 
The FGD by-product gypsum slurries are preferably filtered and received for 
use in gypsum wallboard production at a free moisture content of about 
15%, and a combined water content of about 20%, based on solids content, 
and a purity of from 94% to 98%. Impurities include inert materials, such 
as SiO.sub.2, and CaCO.sub.3. Soluble ion impurities such as Mg.sup.++, 
Na.sup.+, Cl.sup.-, SO.sub.3.sup.- must be kept to a very low prescribed 
level in order not to adversely affect the final product. 
The 15% free moisture filter case is continuously fed to a flash dryer such 
as a Raymond Cage Mill Flash Drying System sold by C-E Raymond Combustion 
Engineering Inc. of Chicago, Ill. This drying system can, with the largest 
available unit, dry the 15% filter cake at a rate of 50 tons/hour. The 
thermal efficiency of this dryer is about 82%. 
The Cage Mill Flash Drying System includes a cage mill which disintegrates 
agglomerated material without any grinding or particle breakdown. The damp 
FGD gypsum is fed into a supply of air at about 900.degree. F. as the air 
enters the cage mill at the central axis. The exit air temperature is 
185.degree.-200.degree. F. The cage mill has a plurality of rotating 
impellers which break up the agglomerated damp gypsum prior to its being 
hot air conveyed to a cyclone separator. Dried FGD gypsum is discharged 
from the cyclone separator through a rotating air lock and the hot 
conveying air is directed to a secondary collector to remove substantially 
all of the solids still present. 
The dried FGD gypsum has substantially no free moisture and is still 
substantially free of any calcined gypsum. 
This dried FGD gypsum is then conveyed to a flash calciner as disclosed in 
U.S. Pat No. 3,956,456, wherein a process is described for calcining a 
gypsum raw material which has been pre-ground, as is common when calcining 
natural gypsum. 
The dried, but not pre-ground, FGD gypsum is continuously fed to a 
peripherally confined cylindrical heating zone by means of a moist heated 
gas stream moving tangentially into the upper part of the heating zone. 
The FGD gypsum then moves through the heating zone in a spirally moving 
heated gas stream, which is repeatedly replenished by additional heated 
gas streams entering the heating zone through a plurality of entry 
orifices arranged essentially tangential to and along the cylindrical 
periphery of the heating zone and distributed generally throughout the 
entire surface of the cylinder. The FGD gypsum is heated by the heated gas 
to a temperature which is sufficient to remove part of the chemically 
bound water therein. The moist heated gases are then drawn off from the 
heating zone in the vicinity of the central axis and about 90% of the 
exhaust gases are heated anew to a higher temperature. Calcined FGD 
gypsum, mostly in the form of calcium sulfate hemihydrate, is removed from 
the heating zone at the bottom. 
A portion or all of this flash calcined FGD gypsum is then screw conveyed 
to the inlet of a centrifugal impact mill, such as the Entoleter 
CentriMil.TM. mill, sold by Entoleter, Inc. of New Haven, CT, preferably 
the series 42 mill. The mill employs a rotor, rotating at 1100-2000 rpm, 
which has projecting out from the rotor a plurality of pins which impact 
the powder fed thereto, and propel the powder against a target area where 
final particle size reduction takes place. The target area has a companion 
row of stationary pins formed into the mill housing liner. The processed 
material then spirals to the bottom of the conical discharge hopper and 
out to bins or conveyors. 
Preferably about 25%-50% of the flash calcined FGD gypsum is impact milled, 
followed by thorough blending with the 50%-75% of material not milled. A 
Fisher surface area of the 100% milled product or of the blended product 
has been found to necessarily be at least about 2500 cm.sup.2 /gm in order 
to be usable to make good quality lightweight wallboard at normal 
production rates. The calcined FGD gypsum, prior to post-grinding has a 
surface area of about 800-1700 cm.sup.2 /gm. By impact milling 25%-50% of 
the calcined FGD gypsum to a fineness of about at least 4000 cm.sup.2 /gm 
and then blending it with 50%-75% unground calcined FGD gypsum, a final 
blend of at least 2500 cm.sup.2 /gm is provided at a lower cost. Grinding 
25%-50% of the material to 4000 cm.sup.2 /gm has been found to be less 
costly, based on equipment and power costs, than grinding 100% of the 
material to &gt;2500 cm.sup.2 /gm. 
The flash calcined FGD gypsum consistency is unexpectedly reduced by the 
post-grinding from about 70 cc/100 gms to about 65 cc/100 gms, providing a 
material which requires less water added to produce a workable slurry for 
the manufacture of wallboard, and thus reducing the cost of drying 
wallboard produced with the post-ground flash calcined FGD gypsum. 
Gypsum wallboard is produced from a paper-faced aqueous slurry of the flash 
calcined FGD gypsum in the standard process as is well known. A four-foot 
wide, half-inch wallboard can be produced with very good quality at 
weights of about 1600 pounds per thousand square feet. 
This wallboard is produced by combining the flash calcined, post-ground FGD 
hemihydrate gypsum with water, an aqueous lightweight foam solution or 
other lightweight aggregate and other well known additives in a standard 
pin mixer, depositing the mixed slurry on the inner surface of a paper 
face sheet, folding the edges of the face sheet upwardly and inwardly, and 
placing a paper back sheet on the top surface, as the combined materials 
pass under a master roll which forms the composite into a continuous web 
wallboard of desired uniform thickness. This continuous web is conveyed 
for a time sufficient for hardening of the core prior to being cut into 
standard board lengths and dried in a high temperature oven. 
The amount of water that needs to be removed in the drying oven is markedly 
reduced beyond what would be expected by the present invention. The flash 
calcined FGD gypsum, prior to post-grinding, has an ultimate water demand 
9% lower than flash calcined natural gypsum. The water demand of the flash 
calcined FGD gypsum is further reduced to 14% lower than flash calcined 
natural gypsum by the post-grinding process. The natural flash calcined 
gypsum to which the comparison is made is a standard production material 
which was produced using a standard precalcination grinding without any 
post-calcination grinding. Natural gypsum always requires precalcination 
grinding because of the nature of the source, which is crushed rocks in 
particle size from fine dust to small rocks. Gypsum in this form could not 
be flash calcined. 
A still further unexpected advantageous characteristic of the post-ground 
flash calcined FGD gypsum is that particle disintegration, or the degree 
of breakdown as it passes through the pin mixer, is less than natural 
gypsum. As a result, it maintains the very low water demand characteristic 
discussed above. The standard flash calcined natural gypsum, which is 
pre-ground, and not post-ground, breaks down to a greater degree as it 
passes through a pin mixer, and as a result its water demand increases, 
which further increases the cost of drying wallboard made from the natural 
gypsum. 
Major factors in the lower water demand of the FGD gypsum are the uniform 
shape of the individually grown crystals, as received, the shallow depth 
of fissures and the lack of cleavage on the crystal surface and the 
significantly lower surface area of the flash calcined post-ground gypsum 
as compared to natural flash calcined pre-ground gypsum. For example, the 
absolute surface area determined by Nitrogen Absorption-BET method, 
revealed that by-product gypsum has a true surface area about four times 
less than natural gypsum. 
A further advantage provided by the process of the invention is the greatly 
decreased energy requirements for grinding, resulting from being able to 
post-grind in place of pre-grinding. This advantage results from the 
brittle or friable nature of calcined gypsum as compared to the harder 
uncalcined gypsum, and thus the greater ease of breaking up calcined 
gypsum particles, to smaller size particles. 
Having completed a detailed disclosure of a preferred embodiment of my 
invention so that those skilled in the art may practice the same, I 
contemplate that variations may be made without departing from the essence 
of the invention or the scope of the appended claims.