Abstract: An economizer having a structure capable of efficiently warming water and facilitating inspection and cleaning is obtained. In an economizer for warming water by combustion exhaust gas generated by a boiler, to a cylindrical water pipe in which an inflow port and an outflow port are formed on a side surface and through which the water passes, a plurality of gas pipes erected for circulating the combustion exhaust gas are arranged in corresponding fan-shaped portions of the water pipe. The combustion exhaust gas introduced from a bottom surface side of the water pipe folds back at an upper part of the water pipe and flows downward, then folds back at a lower part of the water pipe, flows upward, and flows out from an upper surface side of the water pipe, whereby the plurality of gas pipes efficiently warms the water in the water pipe.

Abstract: The present invention is to provide a method for treating sludge as a by-product of a process of tin electroplating a steel plate by means of a plating bath containing haloid ions, together with an equipment therefor. More specifically, by immersing the sludge in water prior to filtration, the residual sludge is subjected to alkali hydrolysis in the presence or absence of the ions of an alkali earth metal within a temperature range of 100.degree. to 400.degree. C., whereby cyanide compounds and fluoride compounds contained therein under environmental regulations are prepared into environmentally non-hazardous matters. By adjusting the filtrate to pH 7.5 to 10, the precipitate with the principal component of tin hydroxide is deposited to recover the tin components contained in the sludge. Furthermore, the recovered tin is refined into the metal tin, for recycling and use as the electrode by the plating.