Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
_ _ _
This in~ention relate!s to the calcina-tion of
yypsum and the production of g~psum wal:Lboarcl. More
particularly, this invention rela-tes to a novel method
and compositions for reduciny the water demand o gyps~rn
plaster which is used in produciny gypsum wallboard
thereby permitting reduced wallboard drying time,
increased wallboard production rates and lower clryer
energy costs per unit area of wallboard.
BACKGROUND OF THE INVENTION
Calcium sulphate hemihydrate (plaster), which
is the core material for gypsum wallboard, is produced
by the process of calcining powdered gypsum rock. A
number of processes and apparatus for continuous
calcination of powdered gypsum rock are commonly used in
the industry. One process and apparatus for continuous
calcination of powdered gypsum rock are described in
Canadian Patents 779,217 and 779,218, both issued on
February 27, 1968 in the name of R.C. Blair, granted to
the British Plaster Board (Holdings) Limited.
Alternative equipment and process for -the
continuous calcining process are disclosed in U.S.
Patent No. 4,052,149 granted on October 4, 1977 to
National Gypsum Company. The continuous calciner has
two coaxial cylindrical walls and provides for heating
air within the inner cylinder and then directing -the
air, for conveying and heating ~ine gypsum particles,
along a spiral path between the walls of -the two coaxial
cylinders. ~he walls extend from one end of the
calciner to an exi-t at the second end.
llhe resultant continuousl.y calcined p]aster i.s
commonly usecl in the form of an aqueous slurry, to
produce gypsum walIboard. rrhe aqueous plaster slur.ry is
spread in a con-tinuous pre-determined arnoun-t between -two
parallel con-tinuollsly mov:ing paper sheets and allowed to
set to a hardened condition. The h~rde~e~ wet wallboard
is then cut to form boards o:E prescr:ibed length, after
which they are passed through an oven or kiln to be
dried until the excess water ;n -the gypsum board has
evaporatecl.
Usually, a large excess of water has to be
aclded to the calcined plaster -to ensure that suffic;ent
slurry consistency or :Eluidity is obtained to allow a
rapicl and even spreading of the sl~lrry across the width
between the two continuous moving paper .sheets. The
amount of water required by a given plaster to obtain a
plaster slurry mix of a standard consistency is known as
the water demand.
With rapidly escalating energy prices, the
cost of drying wallboard to remove the excess water has
become a major undesirable factor in the overall cost of
the wallboard manufacturing process. It follows that a
lower plaster water demand reduces the excess water used
in the wallboard production process and the energy
required to drive off the excess water is reduced. With
reduced total water requirements for a given boardline
production speed, the wallboard can be dried at lower
temperatures thereby desirably decreasing energy usage
and wallboard drying costs per unit area.
Lower drying temperatures during the drying
process 'have the added ad~antage of reclucing -~he
incidence of w~llboard edge anrl end b-lrning, which is a
major quality control problem :Leading to a substandard
wallboard product. This edge burning problem is
particularly prevalent in modern high speed wallboard
manuEacturing plants where elevated dryer temperatures
are normally required in order -to enahle -the wallboard
to be dried at a rate corresponding to the wallboard
produc-t,ion line speed.
A further advantage is realized if the
wallboard dryer is operating at maximum drying capacity
for a process uslng a normal water demand plas-ter
slurry. In this case the drying stage is the rate
limiting factor governing the overall speed of the
wallboard manufacturing process. Clearly a lower water
demand plaster increases the dryer capacity potential
and permits a boardline speed increase thereby improving
wallboard production output.
~arious ways to reduce the plaster water
demand are known by the gypsum industry. One is the
process o~ aridization, in which a gypsum plaster o~
reduced water demand is obtained by calcination in the
presence of a salt or other compound of high affinity
for water (a so~called 'laridizing agent"). This process
is described in Canadian Patent No. 999,118, granted
November 2, 1976 to BPB Industries Limited. The
preferred aridizing agents claimed in this patent are
water-soluble inorganic chlorides such as calcium
chloride. Unfortunately, aridized plasters produce
undesirable side effects in a wallboard manufacturing
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~ .bj ~
process an~l :in the Einlshed wal.l.boclr-1 p:roduct. For
these reasons, the proce.ss of plaster ari~ization is no-t
generally uLled in the wal.lboa:rd rnanufac-turing ;ndustr~.
Another rnethod o-E reducing water demand
and -thereby increasing the -Fluidity of plaster slurries
is through the addi-tion of water reducing agents. For
several years, it has been a common industry practice -to
use na-turally occurring lignin by-produc-t.s of -the pulp
and paper industry as water reducing agents for the
plaster slurry. Lignosulphonate products known by the
trade marks I,ignosol SF and Orzan G are typical.
materials of this type. The major disadvantage with
these products is that generally they have a retarding
effect on the gypsum setting rate and tend to decrease
gypsum board core strength, both oE which are
detrimental to the manufacture of good quality ~ypsum
wallboard.
Another family of water reducing agents is
disclosed in U.S. Patent No. 4,184,887, granted January
22, 1980 to United States Gypsum Company. This patent
discloses the use of a potassium salt o:E a condensation
product of naphthalene and sulfonic acid as a water
reducing agent for a plaster compositon used in making
cast or molded products.
SUMMARY OF THE INVENTION
Within the gypsum wallboard industry, it is
known that the water demand of a plaster slurry can be
reduced whilst maintaining slurry fluidity, by the
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addition of a synthetic chem:ical di.s~e:rc;ant
(superplas-ticizer~ to the sl~.lrry aE-tcr -the calc.inlng
process.
Superplasticlzers are na-tural, or synthetic
polvmeric, dispersing agents which have found
application in the concrete inclustry by improving the
workability or fluidi-ty of cemen-t aclmix-ture.s.
Superplasticize:r.s appear -to exer-t -their action
by decreasing the surface tension of water, increasing
the surface charge o~ the sol.id particles, or producing
a lubrica-ting film at the partic].e surface.
Superplasticizers of -the synthe-tic type may be
classified into -two main types:
Type 1 - Sulphonated naphthalene-formaldehyde
condensates.
Type 2 - Sulphonated melamine-formaldehyde
conclensates.
Type 1 superplasticizers sold under the trade
marks LOMAR D (a~ailable from Diamond ShamrocX
Corporation) or DARVAN ~6 (available from R.T.
Vanderbilt Company) are usually added to the gypsum
plaster after calcination in a concen-tra-tion ranging
from about 0.01 to about 1 weigh-t percent of the plaster
to reduce water demand. Similarly, a Type 2 super-
plasticize.r sold under the trade mark MELMET L10 can be
added to the plaster after the continuous calcination
process to reduce water demand.
t~
Une~pectedly, it has now been found tha-t the
reductlon in plas-ter water demand can be significantly
improved by adding -the superplasticizer -to the gypsum
before or during calcinakion ra-ther than adcling it after
calcination. E'urther, -the ]ow water demand plaster
produced by in situ calcination with a superplasticizer
surprisingly does not significantly affect the sekting
characteristics of the aqueous plaster slurry, nor are
the resultant wallboard core properties adversely
affected.
The invention i5 directed to a method of
reducing water demand of gypsum plaster produced from
powdered gypsum rock comprising adding a water demand
reducing agent, such as a superplasticizer, to the
powdered gypsum rock before or during calcination of the
powdered gypsum rock to produce gypsum plaster.
It is understood that the applicant's
invention has application in eithar batch or continuous
gypsum plaster preparing operations.
me gypsum plaster of the invention may be
used in -the production of gypsum wallboard.
In the invention, the water demand reducing
agent may be a synthetic polymeric dispersing agent.
In -the invention, the superplasticizer may be
selected from the group consisting of sulphonated
naphthalene-~ormaldehyde condensates and sulphonated
melamine-formaldehyde condensates.
In the invention, the superplasticizer may be
selected Erom the group of substances identified by the
trade marks LOMAR D and DARVAN ~6.
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In the invention, -the superp~lastici~er may be
added to -the powclere-l gyps~lrrl rock in a concentration
ranging from about 0.01 to abou-t 1.0 percent by weiyht
of the powdered gypsum rock.
In the inven-tion, -the superplas-ticizer rnay be
added to the powdered gypswn rock in a concentra-tion
ranging from abou-t 0.05 to about 0.5 percent by weight
of -the powdered gypsum rock.
The application is al.so ~irected to a
superplasticizer composition useful for adding to
powdered gypsurn rock prior to or during calcina-tiorl oE
-the gypsum rock into gypsum plas-ter in gypsum wallboard
selected from the group consisting of sulphonated
naph-thalene formaldehyde condensates and sulphonated
melamine~formaldehyde condensates.
The composition may be selected particularly
from -the group of substances identified by the trade
marks LOMAR D and DARVAN #6.
DRAWINGS
In the drawings:
FIGURE 1 illustrates in graphical form -the
effect of superplasticizer LOMAR D in powder form added
to gypsum plaster,
FIGURE 2 illustrates in graphical form the
effec-t of superplasticizer LOMAR D in aqueous form added
to gypsum plaster; and
FIGURE 3 illustrates in graphical form the
effect of superplasticizer DARVAN #6 added to gypsum
plaster.
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DETACL,ED D~SC~IPTION OF THE INVE~TIO~
Con-tinuous calcina-tion experimen-ts were
carried ou-t in a laboratory ket-t:Le appara-tus which is
essentially a scaled down model oE -the ket-tle design
described in detail in the aforemerltioned Canadian
Patents Nos. 779,217 and 779,218, R.C. ~lair.
The laboratory me-thod comprised dry blending
various weigh-ts oE superplasticizer with ground gypsum
using a Pa-tterson-Kelly twin shell dry blending
apparatus. Each mi~ture was then calcined continuously
in the labora-tory kettle appara-tus using a constant gas
firing rate and a calcina-tion temperature of 147 + 1C.
The plaster produced by in situ calcination
with a superplasticizer was analyzed -to determine:
a) l'he chemical analysis of the plaster;
b) The effect on the fluidity of the aqueous
plas-ter slurry,
c) The effec-t on the plas-ter water demand;
and
d) The percentage of water reduction in the
aqueous plaster slurry a-t a s-tandard
consistency.
A calcined plaster produced without
superplasticizer addi-tion and a plaster containing
superplaticizers added after calcina-tion were used as
controls.
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TABLE 1
LOMAR D (POWDER FORM) - SUPERPLASTICIZER ADDITION TO GYPSUM PLASTER
__ ~ _____________ __ _ ._ ____
~ ___.____ . ___
Che nical Analys s, % _ Water Slurry % Wa-ler
Plaster Hemi Soluble Res;dual Demand Fluidity ~eduction
Type Hydrate Anhydrite Gypsum mls/100 g mm spread a-t std
Consistency
~__ _ .. . _ _ .. .. _ _ __ _
Normal
Plaster
without
addition o
superplast-
icizer 64.6 7.1 2.5 7068 __
_ __ _ _ . _._ .. __
Normal
Plas-ter
LOMAR D
superplast-
icizer
adcled after
calcination
0.05%
LOMAR D* 64.6 7.1 2.5 6779 6.
0.1~
LOMAR D* 64.6 7.1 2.5 6688 11.1
0.15%
LOMAR D* 64.6 7~1 2.5 6493 12.6
Plaster
with
LOMAR D
superplast-
icizer
added
be~ore
calcination
0.05%
LOMAR D* 64.3 7.9 2.3 6684 8.9
0.1%
LOMAR D* 62.6 9.5 1.0 6494 16.3
0.15%
LOMAR D* 64.6 7O5 2.5 6210721.6
* LOMAR D Powder Analysis:
84% Naphthalene sul.phonate polymer
11% Sodium sulphate
5% Free moisture
TABLF. 2
LOIYAR D (SOLUTION E'ORM) - ~SUPERPLA ~ _ _
~ ______ ____. _______
Chemica]. Analysis, ~ Wa-ter Slurry % Water
Plaster Hemi Solub.Le Resi~lual ~emand E'luidity Reduction
Type Hydrate Anhydrite Gypsum mls/100 g mm spread at std
Consistenc
_ . ._ . . _ __ _____ _ .____
Normal
Plaster
without
addition of
superplast-
icizer 68.0 5.9 2.8 71 62 __
._ . ~ _ _ . _ ____ . _____ _____
Normal
Plaster
LOMAR D
superplast-
icizer
added after
calcination
0.05%
LOMAR D* 68.0 5.9 2.8 70 66 4.6
0.1~
LOMAR D* 68.0 5.9 2.8 69 72 7.2
0.2~
LOMAR D* 68.0 5.9 2.8 67 80 11.8
. . _ _ ___ _ ~ _ _ _. _
Plaster
with
LOMAR D
superplast-
icizer
added
before
calcination
0.05~
LOMAR D* 67.3 6.6 2.9 70 72 6.7
0.1%
LOMAR D* - 65.8 7.0 3.3 68 89 12.8
0.2%
LOMAR D* 59.5 7.8 3.1 65 100 16.4
_ _ ~ . . _ . _ ._ _ _
* LOMAR D Aqueous Solution Analysis:
33% Napthalene sulphona-te polymer
<1% Sodium sulphate
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[A~ 3
DA VA 1~6*_= _UPl-,RP:[.~S'['IC Y.~.R Al>l)ITION To_ YP~SI)M PLASTER
_.___ _ __ _____ __ _._ ~ _, __ _______ _ .__ __ _
_ Chemical Analysis, %_ _ Wa-ter Slurry ~ Water
Plas-ter Herni Soluble ~esidual Demand Fluidi-ty Keduc-tion
Type Hydrate Anhydrite Gypsum mls/100 g mm spread at std
Consistency
_ _ _ _ .. ___ ___.__ _ ___ _ __ ___._ __ _ .______
Norlna 1
Plaster
without
addition of
superplast-
icizer 64.5 7.7 2.1 68 74
___ ___ .__ _ ____ ___ __ __ ____ _____ _ _
Normal
Plaster
DARVAN #6*
superplast-
icizer
added after
calcinatior
0.05~
DARVAN #6* 64.5 7.7 2.1 66 82 4.9
0.1%
DARVAN #6* 64.5 7.7 2.1 65 87 8.1
0.15%
DARVAN #6*64.5 7.7 2.1 64 90 9.7
_ ,. _ _
Plaster
DARVAN #6*
superplast-
icizer
added
before
cal.cination
0.05%
DARVAN #6*59.2 11.7 1.1 64 94 11.9
0.1%
DARVAN #6*64.4 6.6 2.4 63 96 13.0
0.15%
DARVAN #6*64.1 6.8 _ _ 62 _ _ __
. __
* DARVAN #6 - Polyrnerized alkyl naphthalene sulfonic acid, sodium salt
a ~
Typical results :Eor LOMAR D Type l super-
plasticizer are shown in TA~LES l and 2, which c1early
demonstrate the particular eEf/3ctiveness of the
superplasticizers added before or du:ring calcination in
comparison with addition after calcination. Adding the
superplasticizer before or during calcination unexpect-
edly produces plasters wi-th a water demand lower than
plasters in which the superplasticizer was added after
calcination. The results indicate -that the LOMAR D
aqueous solution having only 33~ superplasticizer
reduced wa-ter demand more effectively on an equivalent
basis than the LOMAR D powder.
The surprising efEec-t of be~ore calcination
compared with after calcina-tion addition of super-
plasticizers to plaster is also graphically demonstrated
in FIGURES l and 2.
A further example of the unexpected advantage
of superplasticizer add~tion before or during
calcination is presented in TABLE 3 and F'IGURE 3. The
superplasticizer used in this case was the powder sold
under the trade mark DARVAN ~6.
Surprisingly, the superplasticizer did not
lose effectiveness by being subjected to the conditions
of the calcination process. Moreover, the fact that the
performance of the superplasticizer was superior when
added before or during ~alcination compared to adding
the superplasticizer after calcination was even more
surprising. While the applicants do not wish to be
bound to any theories, it appears from preliminary
testing that the superplasticizer is advan-tageously
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modified by the cond:itions of the calcination process,
which is completely unexpected. I-t is apparent Erom the
foregoing d~-ta tha-t regardless of whether the
superplasticizer ls modiEied or not, the performance of
the superplasticizer when addec1 beEore or during
calcination is distinctly improved.
As will be apparent to those skilled in the
art in the light of -the foregoing disclosure, many
alterations and modifications are possible in the
practice of this invention without departing from the
spirit or scope thereof. Accordingly, the scope of the
invention is to be construed in accordance with the
substance defined by the following claims.
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