Abstract
The industrial refining of kaolin involves the removal of iron oxides and hydroxides along with other impurities that cause discoloration of the final product and depreciate its commercial value, particularly undesirable if destined to the paper industry. The chemical leaching in the industrial processing requires treatments with sodium hyposulfite, metallic zinc, or sulfuric and phosphoric acids, in order to reduce, dissolve and remove ferruginous compounds. To mitigate the environmental impact, the acidic effluent from the leaching process must be neutralized, usually with calcium oxide. The resulting solid residue contains phosphorous, zinc, and calcium, among other essential nutrients for plant growth, suggesting its use as a macro and micronutrient source. Samples of such a solid industrial residue were used here to evaluate their potential as soil fertilizer in an incubation greenhouse experiment with two soil samples (clayey and medium-textured). The small pH shift generated by applying the residue to the soil was not a limiting factor for its use in agriculture. The evolution of the concentrations of exchangeable calcium, and phosphorous and zinc extractability by Mehlich-1 extractant during the incubation period confirms the potential use of this industrial residue as agricultural fertilizer.
Highlights
Kaolin is a clay consisting essentially of minerals of the kaolinite (ideal formula, Al4(Si4O10)(OH)8) group
These methods are less efficient than differential dissolution treatments, as for example by leaching the ore chemically with sodium hyposulfite or metallic zinc and sulfuric and phosphoric acids in the industrial processing (Trawinski, 1980). This is the procedure adopted by Caolin Azzi Ltda, a mining industry located in the city of Mar de Espanha (21 ° 52 ’ 02 ’’ S, 43 ° 00 ’ 35 ’’ W), state of Minas Gerais, Brazil
At Caolin Azzi Ltda, the acidic effluent from the leaching process is initially neutralized with calcium oxide, in order to mitigate these hazardous effects, following the procedure described by Silva (1997)
Summary
Kaolin is a clay consisting essentially of minerals of the kaolinite (ideal formula, Al4(Si4O10)(OH)8) group. Physical or mechanical methods, such as dispersion or magnetic separation, to remove iron oxides and hydroxides are not always economically convenient, depending on the destination of the final product, while the efficiency is related to some kaolin characteristics These methods are less efficient than differential dissolution treatments, as for example by leaching the ore chemically with sodium hyposulfite or metallic zinc and sulfuric and phosphoric acids in the industrial processing (Trawinski, 1980). At Caolin Azzi Ltda, the acidic effluent from the leaching process is initially neutralized with calcium oxide, in order to mitigate these hazardous effects, following the procedure described by Silva (1997) This produces a suspension that is disposed in ponds in the surroundings of the industrial plant and left to evaporate under open sky, without any further treatment or environmental protection, as those recommended by Cameselle et al (1995). The main purposes of this study were (i) to carry out soil tests to evaluate the potential of this industrial residue as nutrient source for agricultural purposes and (ii) to indirectly evaluate the environmental impact, when used as soil fertilizer in agricultural areas
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