Preferable examples of the alkali agent as component (c) are hydroxides such as sodium hydroxide, potassium hydroxide, calcium hydroxide and the like, carbonates such as sodium carbonate, potassium carbonate, sesqui-sodium carbonate (sodium sesquicarbonate) and the like, silicates such as sodium silicate, potassium silicate and the like, alkanol amines such as monoethanol amine, 2-amino-2-methyl-1