Problems with Aqueous and Gaseous Waste

In the past it has been common practice to discharge the waste acid directly into the open sea or coastal waters. For a long time the weak acid problem has been the subject of public discussion and criticism. As a result the European Community has decided to stop the discharge of weak acid into open waters until 1993. 

The European titanium dioxide producers have developed different effluent treatment processes to meet the environmental requirements [2.47]. The most important processes are the precipitation of gypsum (CaS04) from the weak acid [2.48] and the concentration and recovery of the free and bound acid. 

In the recycling process both the free and the bound sulfuric acid (as metal sulfates) can be recovered from the weak acid in the calcination furnace (Fig. 17, k) and in metal sulfate calcination (Fig. 18). The process consists of two stages  

2) Thermal decomposition of the metal sulfates and production of sulfuric acid from the resulting sulfur dioxide 

As a result of energy requirements only acid containing 20% H2S04 can be economically recovered by evaporation. The weak acid is concentrated from ca. 20-25% to ca. 28% with minimum heat (i.e., energy) consumption, e.g., by using waste heat from sulfuric acid produced by the contact process [2.49], or from the waste gases from the calcination kilns used in Ti02 production [2.50] (Fig. 18). 

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