CONVERSION OF FLUOROANHYDRITE TO PLASTER

Abstract: Gypsum plaster and gypsum wallboard products can be made from fluoroanhydrite by contacting the fluoroanhydrite with a reactive silica selected from the group consisting essentially of Portland cement; perlite; calcium, sodium or potassium silicates; pyrogenic colloidal silica and diatomaceous earth; and after gradual conversion of the fluoroanhydrite to substantially pure fluorogypsum, the purified gypsum is processed in customary fashion in the industrial manufacturing of plaster products. The reactive silica may be added to the fluoroanhydrite during the gradual aging conversion in water to fluorogypsum; or it may be added during water calcination of the fluorogypsum to stucco; or to the gauging water during gypsum plaster or wallboard product formation from the fluorostucco.

Description: Conversion of Fluoroanhydrite to Plaster Background of the Invention Field of the Invention This invention relates to a process for transforming fluoroanhydrite into gypsum plaster and gypsum wallboard products. Flύororanhydrite is a by-product in the manufacture of hydrogen fluoride from the sulfuric acid treatment of fluo- rite (calcium fluoride) . Historically, the fluoroanhydrite, which is contaminated with sulfuric acid, is neutralized with lime or calcium carbonate additions and allowed to hydrate naturally over a several year period of time while weathering in waste heaps. It had been hoped that upon wea¬ thering such materials would then be usable in the industrial manufacture of various products. This has not proven to be the case. Description of the Prior Art Some early attempts to utilize the naturally weathered materials looked at the relationship of this material to Keenes cement (U.S. 1,304,148) and attempted to convert the OMFI weathering anhydrite material directly to the dihydrate form by the inclusion of various hydration catalysts and acceler¬ ators. Such proposed means to convert this waste product into an industrially usable product have focused upon the quick setting or hardening of the anhydrite content into a dihydrate material in the presence of reactive silica com¬ pounds coupled with pozzolonic materials and lime. See for example U.S. Patent Nos. 2,060,127; 2,606,128; 2,608,491 and the like. There is little demand for such inorganic binders; the primary commercial usage for calcium sulfate materials being in the formation of gypsum wallboard. In addition to residual acids, fluoroanhydrite contains other impurities. Fluoroanhydrite or fluorogypsum can con¬ tain up to 2-3% fluoride, probably derived from unreacted calcium fluoride, together with smaller quantities of unre¬ acted soluble and insoluble silicofluorides. These fluorine impurity species, probably a fluoroaluminum complex anion such as [AlF-H O ] , impede the practical commercial -r -i . conversion of the stockpiled material into calcium sulfate hemihydrate. First, these impurities inhibit the hydration of the fluoroanhydrite to gypsum. Secondly, they raise the calcining temperature for conversion of the fluorogypsum to stucco. Thirdly, they inhibit the setting of the stucco to a degree unreasonable for present day commercial wallboard formation and result in producing gypsum products of poor quality. Some of the residual fluoride species in the weathering stockpile material form compounds which are water soluble and may be washed away; while other species form water insoluble compounds that tend to co-crystallize with the dihydrate crystals being formed on weathering. These lat¬ ter impurities, on attempts to convert the fluorogypsum to hemihydrate, cause extreme difficulties in the calcination conversion to hemihydrate and inhibit response of the formed stucco to set control agents. Further the stucco exhibits slow disintegration when mixed with gauging water. These are significant problems because all gypsum board lines employ set control agents as being critical in production rate and cost of production. Finally, stucco from such material exhi¬ bits low maximum rate of hydration and slow disintegration upon mixing with gauging water. This is critical in deter¬ mining the strength of the set product. Natural gypsum, im¬ mediately after being calcined, characteristically disintegrates to a large degree upon dispersion in water. This disintegration may be measured as a large increase in the particle surface area. When fluoroanhydrite is neutral¬ ized with lime and hydrated, the resulting fluorogypsum it has been found, after being calcined to a fluorostucco, dis¬ integrates only to a small degree upon dispersion, and the degree of disintegration is further reduced upon exposure to humidity such that an aged fluorostucco approaches the unpre¬ dictable, erratic dispersion properties of aged natural stucco unsuitable for building plaster or gypsum wallboard usage. Summary of the Invention It has now been found that by allowing a controlled gradual weathering of stockpiled waste fluoroanhydrite mate¬ rials and calcination to calcium sulfate hemihydrate in the presence of a reactive silica, a truly industrially usable stucco is obtained. The impurities in fluorogypsum are pre¬ sent on the surfaces as well as throughout the crystal. Acid or water washing, particularly with grinding in between wash¬ ings, is effective to remove the surface exposed impurities. Coupling this with calcination and hydration additives to remove co-crystalline impurities gives a combined effect making the material usable. The obtained fluorostucco has satisfactory calcination characteristics, rheological and physical properties customary to plaster made from natural gypsum rock when mixed with water and is suitable for forma¬ tion into gypsum wallboard. A part of the findings of the present invention is that while some of the fluorine and aluminum species -contamination in by-product fluoroanhydrite can bo removed by careful .wash¬ ing during the various conversion stages, complex species are trapped in the growing calcium sulfate crystal. By including reactive silicates during various stages of the processing, the crystallized species can either be inhibited in their growth and/or removed from the calcium sulfate crystals. Employing about 1-10% by weight of an active siliceous mate¬ rial in some manner frees the calcium sulfate from the poisonous fluoro' and aluminum complex impurities that inhibit hydration of the luoroanhydrite to gypsum, that inhibit sub¬ sequent calcination of the fluorogypsum to fluorostucco, that inhibit the setting action and strength development of the fluorostucco product. Description of the Preferred Embodiments The starting material is a residue from the process of manufacturing hydrofluoric acid from fluorospar. Fluoroan¬ hydrite fresh from the reactor may be treated by the process of this invention and then stored for gradual conversion. Alternatively, or in addition, weathered stockpile material containing fluorogypsum as well as fluoroanhydrite may be treated. Alternatively, or in addition, treatment may be car¬ ried out during calcination of the fluorogypsum to fluoro¬ stucco. Thus, in one preferred embodiment, hot fluoroanhy¬ drite directly from the reactor may be blended with the re¬ active siliceous material and passed to briquetting or pel- letizing apparatus for mixing with a suitable binder while being formed into briquettes or pellets. The briquettes may then be warehoused or stockpiled while the conversion to fluorogypsum occurs. Thereafter, the fluorogypsum is calcined to fluorostucco for use as plaster products or subsequent rehydration in gypsum board manufacture " . Suitable active siliceous materials preferably include Portland cement, finely ground expanded perlite, diatomaceous earth, reactive colloidal silica such as Cab-O-Sil® silica or Aerosil® fumed pyrogenic silica and alkaline earth metal silicates such as sodium, potassium and calcium silicate. These are all siliceous products having high surface areas (greater than about 10,000 square centimeters per gram) and having chemically reactive sites due to surface deformaties, such as chemically incomplete silicon dioxide surfaces, missing oxygen atoms in the alkali metal silicates and Port¬ land cement, or stressed crystal configurations in expanded perlite sintered or fumed silicas and diatomaceous earth. Other reactive siliceous products suitable for use in this invention will be evident from this description. Generally inclusion of about 1-10% by weight of the active siliceous material based on the weight of calcium sulfate present in the fluoroanhydrite will produce satisfactory results. Pre¬ ferred amounts of siliceous material are dependent upon the time at which the material is added in the process and the particular siliceous material. Extensive studies on a partially hydrated fluoroanhy- drite-fluorogypsu stockpile material showed the material to be northomogeneous, and calcination to stucco resulted in very slow setting even when accelerators were used. The slow set¬ ting properties were caused by poor disintegration of the stucco upon dispersion in water, and by a chemical inter¬ ference to . the growth of calcium sulfate crystals in the gyp¬ sum phase. In comparison to natural gypsum, calcination of the stockpile materials exhibited higher drag and dump temperatures. The resultant fluorostucco upon mixing with water had a smaller particle size initially, but a larger particle size upon dispersion. While having a surface area initially comparable to natural gypsum, the fluorostucco exhibited a lower surface area upon dispersion with water in a board machine mixer. It is now believed that while some of the contaminant species are water and/or acid soluble and may be washed off the surfaces of the fluoroanhydrite or fluorogypsum, the dif¬ ficult contaminant species are co-crystalline or occluded, i.e., as the fluoroanhydrite is undergoing the transformation to luorogypsum, contaminant ions in the surrounding solution crystallize on the growing gypsum phase and are occluded or co-crystallized in the gypsum matrix. It has now been found that these contaminating ions can be rendered inactive in the solution phase, and then the gypsum which re-crystallizes is relatively free of impurities and has properties similar to those of natural gypsum. The preferred first step for fresh or weathered fluoro¬ anhydrite material is a water or dilute acid wash. For ex¬ ample, it has been found that washing the weathered material twice with water, with a light grinding in between washings, drammatically increased the surface area from about 6,000 2 2 cm /g to 15,900 cm /g; and reduces the set time with a standard amount of accelerator from approximately 12 minutes without washing to 6.3 minutes after the double washing. Apparently, the light grinding and second wash released some of the impurities. The same treatment steps using 32% sul¬ furic acid instead of water gave an even better accelerated set time response of 4 minutes. However, neither of these treatments provided sufficient strength or the dispersion dis¬ integration characteristics of natural gypsum. Further, dur¬ ing calcination these materials were difficult to stir, had a high drag temperature and the calcination temperature was difficult to maintain and not uniform throughout the kettle. SUBSTITUTE SHEET Coupling this step with active siliceous material addition to inhibit co*-crystalline impurities gives a combined effect making the fluoroanhydrite material usable for building plaster and gypsum wallboard production. As one alternative, active siliceous materials may be added to the mixing (gauging) water of fluorostucco. For example, the following were evaluated as slurry additives to fluorostucco obtained by calcining different weathered fluorogypsum samples without any adjustments. The samples were from different locations in the fluorogypsum pile and analysis indicated widely varying degrees of hydration and chemicals content: Slurry Addition Alone Untreated Slurry Vicat _ Temperature Fluorostucco Additive Amount Set Time Rise Set Time Control — — 10.5 minutes 17.0 Na-SiO 1 gram 8.8 14.0 Elapsed time from mixing 50 g. plaster with gauging water to when a 300 g. Vicat needle will not penetrate more than half way (20 + 2mm) into the setting slurry. From the results it is clear this addition rendered the co-crystalline impurity, non-interfering in the through solution setting of stucco recrystallizing as dihydrate. As another alternative, active siliceous materials may be added to the fluorogypsum at the stage of calcination to fluorostucco. The additives were mixed with weathered fluoro¬ gypsum and then kettle calcined under standard conditions. Since kettle calcination itself is a topatactic dehydration, the additives at this point should have a surface effect rather than a crystallographic effect. _OMPI Calcination A d^t^on Additive Amt. Amt. Drag Dump Hydration Analysis* Added Cal. Temp # F Temp β F Hem. Dihyd.Anh. other Untreated - 4 kg 268 293 B3.6 3.7 2.8 9.9 Control Na 40g 4.04kg 273 302 86 2.1 1.7 10.2 sio 3 Diatomaceous earth 80g 4.08kg 262 293 79 3.2 2.8 15 Portland cement 80g 4.08kg 266 293 81.5 2.8 2.9 12.8 From the table it may be seen that the additives did not sig¬ nificantly affect the calcination properties. Diatomaceous earth reduced the drag temperature by a slight amount, but still significantly above that of natural gypsum. Following calcination, the materials were evaluated for setting and disintegration properties of the fluorogypsum calcined with the additives: Setting & Disintegration Properties of Calcination Addition Additive Accelerator Vicat Temp/Rise Set Control Untreated 10 lb/ton 17.0 min 23.5 (w/o accel.) 11.5 18.5 (w/acceler.) Na 2 Si0 3 10 lb/ton 13.8 18.5 10.8 15.0 Diatomaceous 10 lb/ton 19.0 25.0 earth 12.0 19.0 Portland 10 lb/ton 16.2 23.0 cement 10.3 18.0 The subsequent rehydration of stucco when mixed with gauging water is a through solution process thus the dditives having only a surface effect during calcination when cairied through the rehydration also have a cryst l lographic effect. From the table it may be seen that sodium silicate and Portland cement shortened the with accelerator sotting tiπ.cs and were thus found to increase accelerator response of the calcined OMPI EET ' fluorogypsum. Thus, it seems that the silicate active ion appears to have a beneficial effect. The diatomaceous earth increased the disintegration of the treated stucco to a level equal to that of natural land plaster; however, the set time was not affected. Thus, while some of the contaminant species are water and/or acid soluble and may be washed off the surfaces of the fluoroanhydrite or fluorogypsum, the dif icult-to-re ove con¬ taminant species are co-crystalline. As the fluoroanhydrite is undergoing the transformation to fluorogypsum, contaminant ions in the surrounding solution crystallize on the growing gypsum phase and are occluded in the gypsum matrix. The high drag temperature of the fluorogypsum demonstrates the adverse effect of even low concentrations of the impurities in the gypsum lattice. The impurity species, probably a fluoro- aluminum ion complex such as [A1F_(H.>0)3 —2, desensitizes the calcined fluorostucco to the presence of accelerators in the mixing water. Fluoroanhydrate Hydration Additive The additives listed below were mixed with fluoroanhy¬ drite and neutralized gypsum pond water and allowed to weather hydrate over time. When the samples had hydrated to an appreciable extent above 70% gypsum they were kettle cal¬ cined without further additive addition to fluorostucco and the Vicat and temperature rise set times of the materials with and without 10 pounds per ton setting accelerator and gauging water without further additive addition were deter¬ mined: - Untreated CaSiO-, Perlite Portland 1% 2? lft 2* Ceπent 1? Hydration 23% gypsum 41% 59%* 75% 78% 67% Rate a 6 weeks at 4 -Ms at 6 vritsat . wks. at 6 wks. Calcination data Dry Temp. β F 266 C F 266 266 255 257 255 % Hemihydrate 55.9% 72.4 72.42 70.66 61.79 60.39 % Dihydrate -0- -0- -0- 0.82 -0- 2.20 % Anh drite 4.75 3.05 3.05 2.56 11.24 14.78 % Other 39.35 24.53 24.53 25.96 26.97 22.63 Rehydration data Vicat Set (Minutes) w/o accelerator 4.3 4.0 4.0 9.5 9.8 12.5 w/accelerator 3.0 3.0 3.6 6.3 6.3 6.0 Temp. Rise Set (minutes) w/o accelerator 9.0 8.0 8.5 15.0 16.0 23.0 w/accelerator 6.0 5.5 8.0 11.0 13.0 11.0 Dry compressive strength 390 psi 61S 400 490 440 Humidified 90 β F/90% R.H. compressive strength 340 psi <90 « 370 450 400 •Allowed to hydrate at pH 2 in presence of excess sulfuric acid. The above shows Portland cement and perlite additions at this one processing step lowered calcination temperature of the treated materials. The long unaccelerated set times of treated fluorostucco with some of the materials is not a sig¬ nificant problem because virtually all commercial gypsum , board lines employ set control agents. It has now been found that these contaminating ions can be rendered inactive in the solution phase, and then the gypsum which crystallizes out would be relatively free of impurities and have properties similar to natural gypsum. With sufficient treatment, the occlusion of these fluorine and aluminum ion species as the fluoroanhydrite is undergoing transformation to fluorogypsum can bo prevented completely, and treatment in the calcination and/or gauging water hydra¬ tion stages may not be necessary.

Claims: WHAT IS CLAIMED IS: 1. A process for the conversion of luoroanhydrite to a hydrated calcium sulfate product which comprises mixing a form of calcium sulfate from hydrogen fluoride manufacture with water and about 1-10 weight % of active siliceous material and recrystallizing a hydrated calcium sulfate. 2. The process of Claim 1 in which the hydrogen fluoride manufacture calcium sulfate is fluoroanhydrite, and it is recystallized to fluorogypsum. 3. The process of Claim 1 in which the hydrogen fluoride manufacture calcium sulfate is fluorogypsum, and it is recrystallized to fluorostτιcco. 4. The process of Claim 1 in which the hydrogen fluoride manufacture calcium sulfate is fluorostucco, and it is mixed with gauging water to recrystallize a gypsum pro¬ duct. 5. The process of Claim 1 in which said siliceous material is Portland cement. 6. The process of Claim 1 in which said siliceous material is calcium silicate. 7. The process of Claim 1 in which said siliceous material is potassium silicate. 8. The process of Claim 1 in which said siliceous material is sodium silicate. 9. The process of Claim 1 in which said siliceous material is perlite. 10. The process of Claim 1 in which said siliceous material is pyrogenic colloidal silica. 11. The process of Claim 1 in which said siliceous material is diatomaceous earth. ' 12. The process of Claim 1 in which the hydrogen fluoride manufacture calcium sulfate is washed before being mixed with water and siliceous material. 13. The process of Claim 12 in which the calcium sul¬ fate is washed with water. 14. The process of Claim 12 in which the calcium, sul¬ fate is washed with sulfuric acid. • »•••

Inventors: BRUCE ROBERT BYRON, GAYNOR JOHN C, PALMER JAY W, BRUCE, ROBERT, BYRON, GAYNOR, JOHN, C., PALMER, JAY, W.
Applicants: UNITED STATES GYPSUM CO, UNITED STATES GYPSUM COMPANY
Application year: 1982
Main classification: C01F11
IPC codes: C01F11, C04B11, C04B14