Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2001-08-10
2004-03-16
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S775000, C423S171000, C423S555000
Reexamination Certificate
active
06706113
ABSTRACT:
The invention relates to a process for treating a powdered starting material based on natural calcium sulfate (gypsum) or synthesized calcium sulfate (sulfogypsum, phosfogypsum and other by-products of the same type) with a view to preparing a novel hydraulic binder that can be used as a cement, based on very high proportions of anhydrite III or &agr; anhydrite (&agr; in the ASTM classification).
The invention also relates to the product from said process, that can be used as a cement.
Gypsum has a variety of crystalline forms; on a molecular scale, it has a lamallar structure in which a layer of water alternates with two layers of CaSO
4
.
Gypsum has been used for thousands of years to produce plaster, one of the oldest construction materials, known since the 6
th
century BC.
Nowadays, other industries use gypsum, in particular:
for cement production (as a setting regulator);
for agricultural use;
in a variety of industries (chemicals, paper, etc.).
By far the most important use is its involvement in cement and plaster production, by dehydrating gypsum.
While in the cement industry, gypsum is incorporated into clinkering and is burned at a high temperature of the order of 1400° C., when making plaster, the essential principle is to eliminate the water completely or partially from the gypsum—a complex operation that involves crystallization phenomena that are difficult to control.
Several types of calcium sulfate treatment processes have been proposed for preparing plaster. In particular, improved plaster (sometimes designated a plaster) can be prepared that, once hardened, has mechanical characteristics that are far superior to those of normal plaster. The phenomena that occur during such treatments are little understood and in general, the improvement in mechanical performance is attributed to the presence of anhydrite III or &agr; anhydride in the products obtained, without accurately knowing the proportion of that type in the products, nor the conditions which can produce it in a stable and reproducible manner: it only exists in trace amounts.
Traditionally, improved plaster is produced from gypsum by burning under moist condition, in an autoclave, followed by a hot drying stage carried out in a stream of hot, dry air. Burning is carried out in a saturated steam atmosphere at a pressure of the order of 5 to 10 bars for a period of the order of 10 hours.
In order to try to overcome the drawbacks of that traditional process for producing improved plaster (a very expensive process with uncertain reproducibility), other processes have been proposed that attempt to reproduce the essential conditions of the traditional process (moist heat treatment followed by hot air drying) using different means and technologies (French patents FR-A-2 389 855, FR-A-2 445 940, FR-A-2 572 721, U.S. Pat. Nos. 2,269,580, 3,145,980).
The process of the invention was developed from the following observation: when calcium sulfate is treated normally to obtain an “improved plaster”, the product obtained is in fact a mixture of anhydrous forms (&ggr; anhydrite) or hydrated forms (semihydrates, dihydrates, . . . ). The inventor's studies have demonstrated that this state of affairs essentially derives from two factors. Firstly, burning produces &agr; anhydrite and also other forms, and secondly, the product changes after burning, with partial transformation thereof, in particular re-hydration. The essential idea that has resulted in the process of the invention is to produce a stable final product containing a proportion by weight of &agr; anhydrite that is much higher than that contained in known improved plaster; to this end, the structure of the compound obtained after burning is frozen by quenching. This considerably blocks the subsequent transformation of the &agr; anhydrite formed by heat treatment.
PCT patent document PCT/FR96/00622 discloses that the quench operation is preferably carried out so that the material heated by burning is heated to a temperature of less than 100° C. over a period in the range 6 to 12 minutes. It can be achieved using cold dry compressed air injected at a plurality of locations into the moving material, the flow rate of the air being adjusted to obtain a suitable cooling rate.
However, that quench is not sufficiently effective to produce a proportion of anhydrite III or &agr; anhydrite that is really significant. That prior art patent application cannot produce a very high percentage of anhydrite III or &agr; anhydrite, namely 90%, and thus cannot produce a hydraulic binder that can be used as a cement.
The process of the invention can produce such a product.
The aim of the invention is to refine the process to obtain a high proportion of stable, soluble anhydrite III or &agr; anhydrite.
The process of the invention can transform more than 90% of pure calcium sulfate into anhydrite III or &agr; anhydrite.
To this end, the present invention provides a process for synthesizing a hydraulic binder based on natural calcium sulfate (gypsum) or synthetic calcium sulfate (sulphogypsum, phosphogypsum, titanogypsum, etc), consisting in heating the calcium sulfate to form:
a hydraulic binder that can be used as a cement based on anhydrite III or &agr; anhydrite, characterized in that it contains more than 70% of stable, soluble anhydrite III or &agr; anhydrite and in that it consists in carrying out:
a heating or quenching step that brings the temperature of the treated gypsum from ambient temperature to a temperature in the range 220° C. to 350° C. depending on the characteristics of the treated gypsum;
a step for rapidly quenching the product obtained, reducing its temperature from 220° C.-350° C. to less than 80° C. in less than two minutes, with the aim of stabilizing the &agr; anhydrite by crystallographic blocking and fixing.
In one implementation of the process, the heating temperature is 300° C.-310° C.
In a preferred implementation, the temperature of the quench is in the range 40° C. to 50° C.
In a preferred implementation, the temperature is raised over a period of 10 to 40 minutes depending on the nature and grain size of the gypsum.
Prior to treatment, the treated gypsum comprises 0 to 20% of water, and its grain size is in the range 0 to 30 millimeters (mm).
In a preferred implementation, the treated gypsum comprises 5% to 15% of water and its grain size is in the range 0 to 10 mm.
A hydraulic binder that can be used as a cement based on anhydrite III or &agr; anhydrite obtained by the above process is characterized in that it contains more than 70% of stable and soluble anhydrite III or &agr; anhydrite.
A hydraulic binder that can be used as a cement based on anhydrite III or &agr; anhydrite obtained by the above process is characterized in that it contains more than 90% of stable or soluble anhydrite III or &agr; anhydrite.
A hydraulic binder that can be used as a cement based on anhydrite III or &agr; anhydrite obtained by the above process is characterized in that the mechanical strength is:
22 MPa at 24 hours;
30 MPa at 8 days;
more than 40 MPa at 14 days.
The present invention also concerns a hydraulic binder that can be used as a cement obtained by carrying out the above process.
The essential inventive concept of the invention is thus to increase the proportion of &agr; anhydrite in the product, the essential means employed being to limit the change in the product after burning by rapid cooling. To further increase this proportion of &agr; anhydrite, the inventor also applied himself to optimizing the burning operation to obtain the largest possible quantity of this variety following burning.
When heated, gypsum produces a series of hydrated or anhydrous products.
At about 100° C., &agr; or &bgr; semi-hydrates are obtained (depending respectively on whether steam pressure or free air is used) as defined in the reaction:
At about 300° C., anhydrite III or a very soluble but highly unstable anhydrite is obtained which immediately re-hydrates to the semi-hydrate in contact with water vapor:
At about 300° C. for the &agr; semi-hydrate and 350° C. for t
Cohen & Pontani, Lieberman & Pavane
Marcantoni Paul
Societe Europeenne d′Exploitation de Technologies Industri
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