Alkali hydrothermal synthesis of zeolite A using blast furnace (BF) slag was investigated. The preliminary experiment was conducted in use of synthetic slag consisting of SiO 2 , Al 2 O 3 , and CaO powders, and it was made clear that the most optimum slag compositions to synthesize zeolite A were the molar ratio of Si to Al (Si/Al) of 1 and reduction of CaO content down to 15 mass%, and that hydrothermal treating conditions were the temperature range from 328 to 358 K, NaOH solution of 1 M (ϭmol/L) and the ratio of the volume of NaOH solution to total mass of slag (V sol /W slag ) of 15 (mL/g). It was also found that synthetic slag with such a higher content of CaO as 40 % resulted in formation of tobermorite and hydrogarnet. In the experiment using BF slag, zeolite A could be successfully synthesized by optimizing both conditions of the compositions of raw material powders and hydrothermal treatment as noted above, where optimization of the compositions of raw material powders such as Si or Al content were performed by the suitable amount of addition of SiO 2 powder or NaAlO 2 powder as a source of Si and Al. The ball milling type reaction vessel containing numerous small SiC balls which was first adopted in this hydrothermal treating study was confirmed to be very effective for acceleration of synthetic reaction rate, shortening markedly the time period needed for fully synthesis of zeolite A. Temperature dependence of heat capacity of zeolite A powder synthesized in use of BF slag was measured after absorption of vapor at the ambient temperature, showing endothermic behavior with the peak at the temperature of around 473 K.

The elution and recovery of Ca from blast furnace slag and the alkali hydrothermal synthesis of zeolite A using the residue from the elution treatment were investigated using a ball-milling-type reaction vessel. The superior elution was obtained by using citric and formic acid solutions from which Ca could be selectively eluted from the slag. Whereas hydrochloric acid solution and ion-exchanged water were ineffective for Ca elution. Ca ions eluted in the solution were recovered as calcium formate by evaporation of water. From the formic acid eluted slag residue after three elution treatment cycles of the slag for 7.2 ks, zeolite A was synthesized in 1 mol/L NaOH solution with a reaction time of 86.3 ks at 343 K, without the addition of any other materials.

The alkali hydrothermal synthesis of zeolite A using oxide by-products such as blast furnace (BF) slag and aluminum dross was investigated. Na-P1 (Na 6 Al 6 Si 6 O 32 · 12H 2 O) was synthesized directly in NaOH aqueous solution for 86.4 ks at 373 K using BF slag, Al 2 O 3 , and SiO 2 . On the other hand, zeolite A was successfully synthesized by a two-step method including an elution treatment of Al 2 O 3 . Under optimal conditions, the formation ratio of zeolite A in various compounds contained in the product was 56 %. The utilization of dross residue squeezed out from the aluminum dross as a raw material to synthesis of zeolite A let this ratio increase to 60 %. The heat of water adsorption of the product synthesized by the two-step method using BF slag, SiO 2 , and aluminum dross residue was measured, and it was 184 J/g, being higher than that of commercial zeolite A.