Compositions: coating or plastic – Coating or plastic compositions – Proteinaceous material containing
Patent
1984-06-06
1986-02-25
Dixon, Jr., William R.
Compositions: coating or plastic
Coating or plastic compositions
Proteinaceous material containing
106102, C04B 754
Patent
active
045727392
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and a plant for producing a dry cement composition which, when mixed with water, gives an immeasurable or low concentration of Cr.sup.+6 (in the following also called "chromate") dissolved in the water.
Cement is normally manufactured by grinding a starting material including cement binder clinker and gypsum, and in some cases also other additives, such as fly ash, slag, puzzolane, and grinding aid, the starting material being continuously supplied to and ground in a cement mill. The conventional cement mill is a ball grinder comprising a rotating drum with very large dimensions, such as a length of 15 m and a diameter of 5 m. In operation such a huge ball grinder generates an excessive amount of heat. In order to prevent the temperature in the mill from exceeding an acceptable level, the mill is normally cooled by spraying or atomizing water into the inner space of the rotating drum. Consequently, the atmosphere within a conventional cement mill has a relatively high temperature (normally 120.degree.-140.degree. C.) and a high humidity (about 0.3 kg water vapour per kg air). Thus, the cement mterial is exposed to rather vigorous conditions when treated in the cement mill. Furthermore, during the manufacturing process ground cement composition is normally transported by means of pneumatic transporting devices whereby almost each single cement particle is brought into intimate contact with the transporting air.
It has been known for a long time that cements contain chromium compounds which, when cements are mixed with water, appear as Cr.sup.+6 dissolved in the water, and that this dissolved chromate is objectionable from a health point of view, because it may cause eczema in persons working with the cement-water mixtures when it comes into contact with their skin.
It is known to reduce the concentration of dissolved chromate in a cement-water mixture and in wet concrete by adding ferrous sulfate during the mixing process for producing said mixture. The ferrous ion of ferrous sulfate reduces the Cr.sup.+6 to Cr.sup.+3 which is only slightly soluble in cement-water mixtures. In "Annex 3 to circular letter 15" issued by CEMBUREAU (European Cement Association), Paris, and dated 1st Mar., 1979, it is proposed to dissolve the ferrous sulfate as a 20% solution before the addition. Said annex also describes an experiment in which it was attempted to add the ferrous sulfate solution during the manufacturing process of the cement. The ferrous sulfate was sprayed into the hot and humid clinker in a cement mill together with gypsum. The annex concludes that hardly any chromate was then reduced, even if double the normal dose of ferrous sulfate was added. A better, but still unsatisfactory result was obtained by heating the clinker and gypsum to 140.degree. C. after milling and spraying a solution of ferrous sulfate on the hot cement. Furthermore, in a paper issued by CEMENTA AB and dated 13th Oct., 1977 the authors of the above publication state that ferrous sulfate cannot be added prior to grinding in a cement factory because the environment in the mill causes iron(II) sulfate to oxidize into inactive iron(III) sulfate, and it is concluded that no practical possibility of eliminating chromium compounds in the manufacture of cement exists.
U.S. Pat. No. 3,425,892 issued on 4th Feb., 1969, discloses a cement composition containing 0.1-5% of ferrous sulfate for the purpose of retarding the hardening rate of the cement composition. Said U.S. patent does not at all relate to reduction of water-soluble chromate in cement mixtures.
It has now been found that contrary to the teachings of the above mentioned publications from CEMBUREAU and CEMENTA it is possible to reduce the content of dissolved chromate in a cement-water mixture and in wet concrete by adding ferrous sulfate to the cement during the manufacturing stage, namely if the ferrous sulfate is added in a dry or non-dissolved condition.
Thus, the present invention provides a method of producing a dry cement composition, said
REFERENCES:
patent: 2316039 (1943-04-01), Wilson
patent: 3425892 (1969-02-01), Edelson et al.
patent: 4122059 (1978-10-01), Hansen
patent: 4209335 (1980-06-01), Katayama et al.
Contact Dermatitis 5, 39-42, (1979), Fregert et al.
Chemical Abstracts, vol. 88, No. 20, p. 273, col. 2.
Berufs Derufsdermatosen 20, 5, 238-248 (1972), S. Fregert et al.
"The Technology of Binders", Butt et al., Moscow (1952) pp. 342-343.
Capella Steven
Dixon Jr. William R.
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