For efficient and sustainable steel production, the ash content of the coal needs to be less than 10%. The ash content of coal is mainly due to the presence of alumino-silicates, quartz, silica and other clay minerals that are present in the coal. To remove these ash forming minerals we use a two stage chemical leaching process to bring down the current ash content (Demineralization) from around 40% to about 10%. The two stages are alkali leaching followed by acid leaching. We use Sodium Hydroxide (NaOH) during alkai leaching. It reacts with ash forming constituents to form Sodalite. This process consumes a huge quantity of NaOH. We are therefore seeking solutions that will minimize the loss of sodium (Na) without affecting the percentage demineralization.
Chemical demineralization process of coal involves selective removal of inorganic ash bearing minerals by sequential leaching of coal with NaOH followed by HCl as shown in figure 1 below.
Figure 1: Simplified process flow
During alkali leaching, which is carried out at temperatures of 150 – 200 °C, NaOH reacts with alumino-silicates and silica in coal (coal is less than 0.5mm in size) and forms soluble sodium silicate (Na2SiO3) & sodium aluminate (NaAlO2). In the end sodium alumino-silicates (sodalite) is precipitated out on the surface of coal. Sodalite formed during alkali leaching gets dissolved during the subsequent acid leaching stage, which results in the reduction of mineral matter or ash content in coal. The typical quantities needed are shown in table 1 below.
Table 1: Chemical consumption for treatment of 1 ton of raw coal with 40 % ash content
Dissolution of sodalite in acidic solution results in loss of sodium in the form of sodium chloride. The cost of recovering NaOH from NaCl that is present in the spent acid solution is very expensive. This loss of sodium increases the operating cost of the coal demineralization process. In order to minimize the loss of sodium (Na), it is required to minimize the formation of sodalite without affecting the percentage demineralization. In this context, the main challenge is to achieve the maximum demineralization (minimum sodalite formation) by keeping the dissolved silicate and aluminate ions in the solution itself. These sodium silicate and aluminates can be readily regenerated to sodium hydroxide through an established process if they exist in the spent alkali solution in ionic form.
We had attempted to separate the sodalite from alkali leached coal via flotation process prior to acid leaching stage with an objective to recover the Na (NaOH) from sodalite. However, it was observed that most of the sodalite formed during alkali leaching was firmly attached to the coal particle surface. Hence, sodalite could not be separated from alkali treated coal with this method.
Increase the efficiency of recovering Na (NaOH) to more than 96%. Currently its 92%. We are looking for solutions that will enable us to increase the efficiency of recovering Na (NaOH) more than 96%, currently its 92%, after the alkali leaching stage by (either of the following):
We are not interested in recovering Na after acid leaching stage.