A fluoride sintered body suitable for a moderator which moderates high-energy neutrons so as to generate neutrons for medical care with which an affected part of the deep part of the body is irradiated to make a tumor extinct comprises MgF.sub.2 of a compact polycrystalline structure having a bulk density of 2.90 g/cm.sup.3 or more and as regards mechanical strengths, a bending strength of 10 MPa or more and a Vickers hardness of 71 or more.

A fluoride sintered body suitable for a moderator which moderates high-energy neutrons so as to generate neutrons for medical care with which an affected part of the deep part of the body is irradiated to make a tumor extinct comprises MgF.sub.2 of a compact polycrystalline structure having a bulk density of 2.90 g/cm.sup.3 or more and as regards mechanical strengths, a bending strength of 10 MPa or more and a Vickers hardness of 71 or more.

A fluoride ion conductor contains rubidium, magnesium, and fluorine. In an average composition of the fluoride ion conductor, the ratio of the number of moles of the magnesium to the total number of moles of the rubidium and the magnesium is less than 0.4.

A fluoride ion conductor contains rubidium, magnesium, and fluorine. In an average composition of the fluoride ion conductor, the ratio of the number of moles of the magnesium to the total number of moles of the rubidium and the magnesium is less than 0.4.

The present invention discloses a method for decomposing medium-/low-grade scheelite, specifically comprising steps of: grinding medium-/low-grade scheelite, decomposing in an autoclave by using sodium phosphate and activated magnesium fluoride as leaching agents, and treating by solid-liquid separation to obtain crude sodium tungstate solution and residue. In this way, the medium-/low-grade scheelite is decomposed. Magnesium chloride is added in a sodium fluoride solution to prepare activated magnesium fluoride as a leaching agent. The present invention has the advantage that the high-efficiency decomposition of medium-/low-grade scheelite can be realized with low consumption of leaching agents, and the leaching cost can be greatly reduced in comparison to the existing decomposition processes using sodium hydroxide and sodium carbonate. This process is short in route, simple in operation, readily available and reliable in production equipment, and easy for industrialization.

A positive electrode active material includes: a particle including a lithium composite oxide; a first layer that is provided on a surface of the particle and includes a lithium composite oxide; and a second layer that is provided on a surface of the first layer. The lithium composite oxide included in the particle and the lithium composite oxide included in the first layer have the same composition or almost the same composition, the second layer includes an oxide or a fluoride, and the lithium composite oxide included in the first layer has lower crystallinity than the lithium composite oxide included in the particle.

A positive electrode active material includes: a particle including a lithium composite oxide; a first layer that is provided on a surface of the particle and includes a lithium composite oxide; and a second layer that is provided on a surface of the first layer. The lithium composite oxide included in the particle and the lithium composite oxide included in the first layer have the same composition or almost the same composition, the second layer includes an oxide or a fluoride, and the lithium composite oxide included in the first layer has lower crystallinity than the lithium composite oxide included in the particle.

The invention relates to a method for obtaining a magnesium fluoride (MgF.sub.2) sol solution, comprising the steps of providing a magnesium alkoxide precursor in a non-aqueous solvent and adding 1.85 to 2.05 molar equivalents of non-aqueous hydrofluoric acid, characterized in that the reaction proceeds in the presence of a second magnesium fluoride precursor selected from the group of salts of strong, volatile acids, such as a chloride, bromide, iodide, nitrate or triflate of magnesium, or of a catalytic amount of a strong, volatile acid; and/or an additive non-magnesium fluoride precursor selected from the group of salts of strong, volatile acids, such as a chloride, bromide, iodide, nitrate or triflate of lithium, antimony, tin calcium, strontium, barium, aluminium, silicium, zirconium, titanium or zinc. The invention further relates to sol solutions, method of applying the sol solutions of the invention to surfaces as a coating, and to antireflective coatings obtained thereby.

A method for treating wastewater containing fluorine is disclosed. The method for treating wastewater includes applying a vacuum to a membrane unit including a membrane having a hollow, injecting wastewater into the membrane unit so that the wastewater contacts an outer surface of the membrane, recovering hydrofluoric acid gas by evaporating hydrofluoric acid (HF) in the wastewater based on a vacuum applied to the inner surface of the membrane and moving the evaporated hydrofluoric acid (HF) to the inner surface through the membrane, injecting sweep gas into the membrane unit to discharge hydrofluoric acid gas remaining on the inner surface of the membrane, and forming a metal fluoride by reacting the recovered hydrofluoric acid gas with a metal oxide or metal hydroxide using a scrubbing process.

A method for treating wastewater containing fluorine is disclosed. The method for treating wastewater includes applying a vacuum to a membrane unit including a membrane having a hollow, injecting wastewater into the membrane unit so that the wastewater contacts an outer surface of the membrane, recovering hydrofluoric acid gas by evaporating hydrofluoric acid (HF) in the wastewater based on a vacuum applied to the inner surface of the membrane and moving the evaporated hydrofluoric acid (HF) to the inner surface through the membrane, injecting sweep gas into the membrane unit to discharge hydrofluoric acid gas remaining on the inner surface of the membrane, and forming a metal fluoride by reacting the recovered hydrofluoric acid gas with a metal oxide or metal hydroxide using a scrubbing process.