Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1997-10-27
2001-01-16
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S775000, C432S011000, C432S018000, C432S118000, C432S147000
Reexamination Certificate
active
06174362
ABSTRACT:
The invention relates to a process for the treatment of a first pulverulent material based on natural calcium sulfate (gypsum) or synthetic calcium sulfate (sulfogypsum, phosphogypsum and other sub-products of the same type) so as to prepare a new hydraulic binder. It relates to an installation permitting the practice of the process, as well as to hydraulic binders adapted to be produced by the process.
Several types of processes for the treatment of calcium sulfate have been proposed to prepare plaster. In particular, it is known to prepare improved plasters (sometimes called “&agr; plasters”) which, once hardened, have much better mechanical characteristics than those of present plasters. The phenomena which take place in the course of treatment are poorly known and the improvement in the mechanical performance is generally attributed to the presence of the crystallographic variety a in the known products, without knowing exactly the proportion of this variety in these products nor the conditions which permit obtaining it in a stable and reproducible manner.
Conventionally, these improved plasters are made from gypsum by subjecting the latter to a heating phase in steam in an autoclave, then to a hot drying phase by a current of hot dry air. The heating takes place 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 an attempt to overcome the drawbacks of this conventional process for the production of improved plaster (extremely costly production, uncertain reproducibility), there have been proposed other processes which in fact attempt to reproduce the essential conditions of the conventional process (steam heating, followed by drying in hot air) by using different means and technologies (patents FR 2,389,855, FR 2,445,940, FR 2,572,721, U.S. Pat. No. 2,269,580, U.S. Pat. No. 3,145,980).
The present invention seeks to provide a new treatment process which leads to a new hydraulic binder which is completely characterized by, and enjoys, once hardened, remarkable performance, in particular fire resistance and mechanical resistance especially when totally immersed.
Another object of the invention is to provide an installation permitting practicing this process under economical conditions.
The process contemplated by the invention to treat a first pulverulent material based on calcium sulfate so as to prepare a new hydraulic binder is characterized in that it combines the two following successive steps:
(a) the pulverulent material is heated to a temperature substantially comprised between 220° C. and 360° C. in the presence of steam under suitable conditions to form principally calcium sulfate of the a variety,
(b) the material thus heated is subjected to thermal quenching consisting in cooling by at least 150° C. in a time less than 15 minutes by means of a cold dry gas injected under pressure into the mist of the latter.
By “cold dry gas” is meant a gas whose temperature is less than about 50° C. (preferably ambient temperature of the order of 20° to 30° C. or a temperature below the latter) and whose water vapor content is less than about 5% (mass of vapor relative to the total mass of the gas) preferably less than 1%.
The process of the invention has been conceived from the following observation: when a calcium sulfate is treated in a usual manner to obtain “improved plaster”, the product obtained is in fact a mixture which contains effectively &agr; anhydrite but in which this variety is present in a very minor proportion and is mixed with other anhydrous forms (&ggr; anhydrite) or hydrated forms (semihydrates, dihydrates, . . . ). The inventor's studies have shown that this results essentially from two factors: heating leading to a anhydrite but also to other forms, and above all an evolution of the product after heating, with partial transformation of the latter particularly by rehydration. The essential idea having led to the process of the invention has been to provide a final stable product containing a weight proportion of &agr; anhydrite much higher than that contained in known improved plasters and, to do this, to fix the structure of the obtained compound at the end of heating by thermal quenching. The final transformation of the &agr; anhydrite formed by heating is thus considerably limited.
The quenching operation is preferably carried out so as to bring the heated material to a temperature below 100° C. in a time comprised between 6 and 12 minutes. It can be carried out by means of cold dry compressed air, injected at several locations into the moving material, the air flow rate being adjusted to obtain the desired speed of cooling.
According to a preferred embodiment of the process, at the end of the heating operation, the heated material is poured from an upper level to a lower level at which it is conveyed by vibration as a bed; the quenching operation is carried out on the one hand by sending a first cold and dry air flow through the curtain of falling material between the upper level and the lower level, and on the other hand by injecting cold and dry air through the bed of material in the course of conveyance at the lower level.
The new hydraulic binder obtained by the mentioned process is a stable mixture of a anhydrite of calcium sulfate and of bassanite, in which the weight proportion of a anhydrite is greater than 35%, and particularly comprised between 40% and 65%; this mixture contains very little or no &ggr; anhydrite.
The composition of this mixture and its high proportion of &agr; anhydrite impart remarkable characteristics to the products obtained by setting of the binder, in particular:
fire resistance: inflammability classed in the category MO according to the Standard NF P 92-507,
compressive strength: between 250 and 300 bars
bending strength: between 55 and 70 bars.
One explanation of this performance takes account in part of the soluble nature of &agr; anhydrite and of the higher specific surfaces obtained with this variety (of the order of 6 m
2
/g) which leads, during use of the binder, to a rapid and homogeneous precipitation. It is to be noted that the products obtained by including conventional fillers in the binder according to the invention, impart remarkable mechanical resistance when immersed in water, much higher than the analogous products obtained with known improved plasters.
Essential inventive concept of the invention has thus been to increase the proportion of &agr; anhydrite in the product, the essential means used consisting in limiting, by quenching, the evolution of the product after heating. To improve still further this proportion of &agr; anhydrite, the inventor is also led to optimize the heating operation so as to obtain the highest possible quantity of this variety at the end of heating.
The following conditions of operations seem to give the best results: first of all, the moisture content of the initial material is determined and is adjusted if needed to a value substantially comprised between 12% and 25%; the heating operation is then carried out by causing the pulverulent material to travel in an agitated bed in a confined gaseous atmosphere containing at least a portion of the water vapor freed from the material, and by heating the whole under conditions suitable to raise the temperature of the gases above the bed of material to a value comprised between 400° and 500° C., and in bringing the mean temperature at the interior of the material to a value higher than 250° C. and below 300° C. The heating is in particular carried out by means of infrared radiant heaters disposed above the bed of material, the emission power of said radiant heaters being adjusted in correlation with the speed of travel of the bed of a material to obtain suitable values of the temperature of the gases above the bed and of the temperature at the interior of the material.
The operating conditions have been defined in a pragmatic manner as a result of long trial and error and tests. A possible explanation of the best results obtained thanks to these operative condi
Marcantoni Paul
Young & Thompson
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