Exploratory study of single and multi-step dissolution-precipitation synthesis of carbonate containing hydrocalumite

Abstract

Hydrocalumite is a calcium/aluminium-based layered double hydroxide. This material has a wide range of applications, such as polymer additives, basic catalysts and water treatment agents. There are a variety of synthesis methods available of which co-precipitation is most widely used. The negative aspects of using most of the developed methods are use of high cost metal salts as reagents. These materials are not only high in cost but are highly corrosive and result in damage of production equipment. A salt containing effluent is produced in these processes that can be harmful to the environment, thus requires treatment before disposal. As the interest in the use of hydrocalumite has increased, there is a need for environmentally friendly and more cost effective synthesis procedures. The aim of this study was to explore different reagents, time and temperature reaction conditions for the dissolution-precipitation synthesis of carbonate-containing hydrocalumite. In this method metal oxides/hydroxides reagents are used which are less corrosive and do not lead to harsh effluent production. This method has two distinct steps namely precursor formation followed by intercalation. In the precursor formation step, it was found that for the reaction between calcium oxide and aluminium hydroxide the conversion of both the calcium and aluminium sources increase as reaction time and temperature increase. It was confirmed that the dissolution of aluminium hydroxide is the limiting step. An aluminium conversion of ~90 % is reached at 80 oC at 6 h reaction time. A further increase in temperature reduces the reaction time immensely, with ~90 % aluminium conversion reached within 30 min at 100 oC and 120 oC. Extending the reaction times beyond these points does not have a significant influence on the conversion of aluminium as it remains constant at ~90 %. Of the various carbonate sources tested for intercalation, sodium carbonate showed the most crystalline hydrocalumite samples. Using sodium bicarbonate as a carbonate source renders similar conversion results to sodium carbonate. When reacting these carbonate sources with the precursor, the conversion to hydrocalumite increased with increasing reaction time and temperature. There is a significant spike in the intensity of the primary hydrocalumite diffraction peak of the sodium carbonate samples at 60 oC and 70 oC for 10 h reaction time; resulting in the highest yield of hydrocalumite. Use of air, CO2 (g), calcite and dry ice CO2 (s) as carbonate sources resulted in poor formation of hydrocalumite. Considering the results of the precursor formation and intercalation, the most promising synthesis conditions is to react calcium oxide and aluminium hydroxide at a temperature of 100 oC for 30 min, where after sodium carbonate should be added to the mixture and reacted for a further 10 h between 60 oC and 70 oC.