Chrysotile asbestos giving Mg₃Si₂O₅ (OH) ₄ as general formula is only one natural resources of fibrous mineral. However, asbestos is attracted recently to be harmful to human body. A fiber of asbestos consists of tubular structure built up by repeating the outer Mg (OH) ₂ layer and the inner Si₂O₅ layer.
In an attempt to improve asbestos for harmless, studies were made of behaviour of Mg²⁺ ion in dissolving of asbestos with acid and surface improvement process to form hydroxyapatite on the surface after dissolving the surface Mg²⁺ ions. The characteristics of both original asbestos and improved asbestos were determined by means of X-ray diffraction, scanning electron microscopy, Infrared spectroscopy, measurement of specific surface area and chemical analysis.The surface layers of Mg (OH) ₂ were dissolved with dilute acid of HCl or H₃PO₄ (0.01-0.50 N) to keep the surface layered structure of Si₂O₅ without destroying even after dissolving. Any clear change was not found from an appearance of the asbestos after dissolving when the dissolved Mg²⁺ is a small amount as 1-10% indissolved-M²⁺/total-M²⁺.Especially, when an amount of the dissolved Mg²⁺ was about 2% to be equivalent to the amount of the monolayer of Mg²⁺ on the surface of asbestos, it was confirmed that the original fibrous form was well kept without seening any destruction.
When the acid-treated asbestos as the above was placed in a solution containing Ca²⁺, Ca²⁺ ions adsorbed easily from the solution to surface sites of the asbestos after removing Mg²⁺ ions. This is thinkable as a kind of exchange reaction. However, the exchanged amount of Ca²⁺decreased with increasing the amount of dissolved Mg²⁺ and the exchange ratio of Ca²⁺ was about 20% in the asbestos removed 10% Mg²⁺. When the asbestos exchanged with Ca²⁺ as the above was placed in Na₂HPO₄ solution, the surface of the asbestos was coated with hydroxyapatite to combine with the surface Ca²⁺ strongly.

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An experiment was carried out to see what in fluences dissolved ion species have on the neutralization of acid waters by crushed limestone, with a main focus on pH alterations caused by addition of Al³⁺ or F^-.
A cubiform limestone with a side of 16mm was put either in hydrochloric acid solutions or in sulfuric acid ones, and they were each stirred at 900rpm, at 25°C.
Neutralization, with Al³⁺ coexistent, is hard to perform beyond pH4, because of Al³⁺'s hydrolytic effects. Almost all Al³⁺ precipitates as hydroxide, but if F^- coexists fluoroaluminate complex ions are formed, and so the Al (OH) ₃ does not reach 70%.
As far as the H₂SO₄-F^- system is concerned, H⁺ and F^- associates with each other, as expressed in the formula H⁺ + F^- _??_ HF, and so sulfuhc acid pH moves up with F^- added. If neutralization gets started, the rise of pH is temp orarily suppressed : Ca²⁺ + 2HF _??_ CaF₂ + 2H⁺.
Neutralization of acid thermal water of Cl^--SO₄^2- system with limestones gave their rate constants in the range of pH 1-4 of the neutralization curve. The apparent activation energy was estimated as 7-9 kJmol^-1 based on these constants. After the neutralization, some precipitations rich with aluminium were found adhering directly onto them, and further gypsum deposited.

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An investigation of gypsum precipitation-dissolution in acid water samples gave certain results about gypsum produced on the industrially used limestone surfaces.
The gypsum solubilities were measured in the range of 25 - 60°C. In H₂SO₄ with concentrations over 5 × 10^-3 mol/l, greater solubilities were observed in higher temperatures, when 0.245g CaSO₄/100g solution was used at 60°C. In acid waters over pH 3, solubilities were almost the same as those in water samples, irrespective of temperature.
The solubility of gypsum is less in CaCl₂ solution than in water. In HCl-CaCl₂ solution thesolubility is affected by CaCl₂ in lower temperatures and by HCl in higher temperatures.With the concentrations of SO₄^2- over 1400mg/l, gypsum precipitation tends to increase rapidly. The produced gypsum, with 60°C, SO₄^2-- 1650mg/l, coats the limestone surfaces closely and perpendicularly in a short crystal-columnar form; with 25°C, SO₄^2- 1500mg/l, in a rough and needle crystal form.

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Hydrated lime (Ca (OH) ₂) granules were fabricated by the reactions of quicklime (CaO) with water, acidic- and basic-solutions (1.0 mol·dm^-3). The granulation of Ca (OH) ₂ powder was performed by spraying the solution over CaO powder in the pan of a tumbling granulator. The conditions suitable for fabricating Ca (OH) ₂ granules with sizes 2-5 mm were as follow : (1) the amount of solution mixed with 30 g of CaO powder was 36 cm³, (2) the declining angle of the pan was 50°, and (3) the rotation rate of the pan was 40 rpm. When Ca (OH) ₂ granules were fabricated by mixing CaO powders with HNO₃ and NH₃ solutions, more than 50 wt% of Ca (OH) ₂ granules were in the range of 2-5 mm. These granules contained not only Ca (OH) ₂ but also small amounts of CaCO₃ : moreover, gypsum (CaSO₄·2H₂O) was formed when CaO powder was granulated with H₂SO₄ solution. The solubilities of granules in citric-acid solution (2%) were as follows : NH₃-added granules > HCl- added granules = HNO₃-added granules ≥ H₂O-added granules > CH₃COOH-added granules ≥ H₂SO₄-added granules. This ordering was opposite to that of the strengths of the resulting granules, except for the case of NH₃-added granules.

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An investigation was made of the optimal condition under which calcium carbonate neutralizes acid water produced by the oxidation of iron sulfide ore.
Iron sulfide is oxidized to sulfuric acid and ferrous sulfate. Through this oxidation from S^2- to SO₄^2-, S₂O₃^2- was found to be intermediately formed. S₂O₃^2-, furthermore, leads the creation of SO₂ and the elemental sulfur, by the redox reaction of S₂O₃^2-.
As for iron sulfude, marcasite decomposes more easily than pyrite. Iron sulfide whose S/Fe mole ratio is lower than 2.00 has a great decomposition rate. It is more efficient and economical, with a smaller amount of calcium carbonate needed, if Fe²⁺ is oxidized to Fe³⁺ through the medium of Thiobacillus ferrooxidans bacteria and then the acid water is neutralized.

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