Limestone and dolomite have many uses for the important materials of industry, but the quality of their manufactures usually cause many troubles on practical use and the causes are often discussed from each standpoint. Therefore, the manufacturers are also making every efforts for the control of manufacturing process and the quality of raw materials.
In order to know the constitution of impurities contained in limestone and dolomite and their act in the manufacturing course, the present authors collected 12 kinds of the low quality limestone and dolomite for sample from the whole country in Japan. First investigated the physical and chemical. properties of sample rocks in detail, next examined the constitution of a difficult soluble impurities separated from sample rocks by acid dissolution process, more analysed the reaction course between the impurities and pure CaCO₃ or MgCO₃ at the high temperatues by X-ray analysis.
The results are summarized as follows; the minute existences of MnO and P₂O₅ as the special components are found 10^-5 degree at all sample rocks by the colorimetric determination. The acid dissolution velocity of rock is remarkably influenced the density of structure and the content of impurity. Above all the higher the content of silica becomes the stronger the strength of structure increases, but the more the acid solubility decreases. According to the acid dissolution method of the silicatic fertilizers, the impurities in sample rocks were separated as a difficult soluble matter from the acid solution and their constitutions were analysed from X-ray diffractometer patterns. The constitution of the impurities extracted from rock were mainly formed with crystalline and amorphous silica, more several clayey minerals attached to them. The particular peaks of quartz for a crystalline silica and them of illite, moscovite, serpentine for clayey minerals were clearly found on the X-ray chart, but their intensities have an effect on the specificity of mineral bed. There, it was distinct that a minute amount of manganse and alkali in the lime manufactures came from the impurities in rock. At the high temperatures from 500°C to 800°C the impurities were increasing acid solubility with separating free Al₂O₃ and Fe₂O₃ because the clayey minerals contained in them dehydrated. But beyond 800°C decreasing acid solubility again because the recrystallization of free R₂O₃ advanced. Pure CaCO₃ was increasing an activity with decomposing to CaO beyond 825°C and rapidly reacting with the impurities. The reaction products were composed of β-2 CaO·SiO₂, α′-2 CaO·SiO₂, 2 CaO·Fe₂O₃ and 3 CaO·Al₂O₃ etc. and easy soluble in hydrochloric acid. Moreover pure MgCO₃ was gradually stabilizing with decomposing to MgO and hardly reacting with the impurities. The reaction products were composed of forsterite, enstatite, ferrite, spinel etc. and difficult soluble in hydrochloric acid.
The above compounds are often found in calcined lime and dolomite and easily separated from the high quality part by screening or blowing. The low quality part separated should be used for a calcium magnesium silicatic fertilizer or a hydraulic cement mortar.

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It is important to utilize by-produced gypsum. But, if the synthetic gypsum is produced economically from waste limestone and sulfuric acid, it is useful in our country. The forms and properties of synthetic calcum sulfate obtained by the reaction of calcite powder and sulfuric acid were studied. The results obtained were summerized as follows :
(1) The heat of formation of anhydrite from CaCO₃ and 98% H₂SO₄ was 21 kcal/mol., and the reaction products were obtained in dry state.
(2) Forms of calcium sulfate obtained by this reaction were affected by the reaction temperature and the concentration of sulfuric acid. Stable anhydrite was obtaincd in the upper zone of the curve showed by the equation (A).
t=254-1.27c+0.0169c²-0.00023c³…… (A)
t : temperature of the reaction in°C
c : concentrations of sulfuric acid in %
(3) The properties of Keen's cement with synthetic anhydrite was very excellent.

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By digesting phosphate rock with sulfuric acid and a great deal of urea, it was found that urea-gypsum and the mixed solution of phosphoric acid and urea were manufactured very easily by the following equation : Ca₃ (PO₄) ₂+3H₂SO₄+12Urea=2H₃PO₄+3CaSO₄·4Urea. However, urea-gypsum was columnar crystal of a shape wherein the length was about five times the width, and the crystals gave an exceedingly sloppy filter cake. By adding a little sodium-alkylbenzenesulfonate to the reacting mixture so that urea-gypsum crystals were produced in a granular form which is an easily filterable type, and recoveries of phosphoric acid in the reacting mixture were about 95%. The continious process of this manufacture can be operated by maintaining the circulation of a part of the filtrate.

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