EFFECT OF ACID CLEANING USING INHIBITOR IN A TUBULAR DIGESTION AND IN A VAPOR RECOMPRESSION EVAPORATOR AT SUMITOMO’S ALUMINA REFINERY

It is very important to keep the heat transfer surface clean to operate the Bayer circuit steadily with low energy consumption. This paper will outline the chemical cleaning of a tubular digester and a falling film evaporator with vapor recompression system which was developed by Sumitomo group.

1. When calcium compounds are added in a digestion step to remove organic compounds and to causticize the liquor, Ca-contained sodalite is formed. The scale is difficult to clean chemically by H₂SO₄ because of insoluble CaSO₄ on the scale during acid cleaning. An inhibitor for HCl cleaning was developed and applied to Sumitomo’s low temperature tubular digester. The equipment material (low carbon steel) is sound after a few hundreds times cleaning.

2. We developed a falling film evaporator with vapor recompression system. Evaporated vapor is recompressed by electric compressor and used as vaporization steam. In this vaporization system, vapor is not exhausted into cooling water as in a conventional multi-stage flash evaporator. Thus the energy consumption is very low compared to a conventional one. As for scaling on the heat transfer surface, after searching for the optimum operation conditions, we clean it chemically using corrosion inhibitor every six month to keep a heat transfer coefficient of 3,000–4,000 J/m²•sec•K. In our plant (60 t/hr) actual energy consumption per ton of water is (0.002t-steam + 37Kw-electric power). It is about 1/5–1/6 of a multi-stage flash evaporator.