Preparation of Sodium aluminate From Basic Aluminium Sulfate

A method for preparing Sodium aluminate from basic aluminum sulfate (BAS) is presented.  The process consists of two steps.  In the first step, BAS was transformed into sodium dawsonite (NaAl∙(OH)2∙CO3) by treating BAS with sodium carbonate aqueous solution at various temperatures and times.  The best experimental conditions for preparing sodium dawsonite were established.  In the second step of this work, Sodium aluminate was obtained by heating sodium dawsonite.  In this case, several samples of sodium dawsonite were heated at different temperatures in the range of 600-1100°C for 30 minutes.  Sodium dawsonite decomposed at 320°C with the evolution of carbon dioxide and water.  At 500°C, a pattern corresponding to a transition alumina was observed by X-ray diffraction.  The first traces of crystalline Sodium aluminate could be detected in the solid heated at 800°C, whereas at 1000°C a mixture of Sodium aluminate and alpha alumina was obtained. Thus, the scheme of phase evolution on progressive heating could be expressed as sodium dawsonite, amorphous, transition alumina (gamma/eta) and crystalline sodium aluminate.  By this method, crystalline sodium aluminate could be obtained under mild conditions by heating sodium dawsonite at 900°C for 30 minutes.

Several methods for preparing solid Sodium aluminate have been developed.  In most methods, an aqueous sodium aluminate solution is prepared in a first step.  Then, the sodium aluminate solution is dried in order to obtain the solid phase.  A typical process for producing aqueous sodium aluminate is by dissolving aluminium hydroxides in a caustic soda solution .  In this case, a suspension of aluminum hydroxide with excess NaOH is prepared. Then, the suspension is passed through heated reaction tubes and the resulting sodium aluminate solution is spray dried.  The product of this process is NaAlO2, NaAlO2∙1.5H2O or NaAlO2∙xH2O.  In another process, sodium aluminate is prepared by solid state reaction of sodium hydroxide and subdivided aluminum hydrate, at a temperature above the melting point of the caustic soda but below 600°C. It has been reported that sodium aluminate could be recovered from sodium dawsonite found in association with oil shales.  In this case, sodium aluminate was obtained by the reaction of homogeneously mixed sodium oxide and aluminum oxide, which were generated during thermal decomposition of sodium dawsonite.

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