The appearances of RHA obtained at different conditions are shown in Fig. 2. RHA obtained at 600 °C is slightly grey in color. And the color of RHA600-2 h is little whiter than RHA600-1 h. It means the residual carbon content of RHA obtained at 600 °C is low which is shown in Table 2. But there are some black particles in RHA700-1 h. It means there is more residual carbon in RHA700-1 h. So the loss on ignition RHA700-1 h is higher which is shown in Table 2. Temperature has great impact on the residual carbon content of RHA. If the temperature is raised to the decomposition temperature of K2O, the unburned carbon surface will get entrapped in the melt. Once carbon is entrapped in the potassium rich melt, it could not be burned completely as it does not contact with air directly [22].