The pH value of coal tar

The reason for adding or removing coal tar is not due to its acidity; rather, it is because the water contained in the tar is actually ammonia water. In this dilute ammonia solution, part of the ammonia exists in the form of ammonium hydroxide, while the other part is in the form of ammonium salts. These fixed ammonium salts include ammonium chloride, ammonium thiocyanate, ammonium sulfate, etc., among which ammonium chloride is the main one. These salts remain in the tar during the final dehydration stage of the tar; when heated to 220–250°C, the fixed ammonium salts decompose into free acids and ammonia. For example: The acid produced by NH4Cl, HCl + NH3 is present in tar and can cause severe corrosion of equipment pipes. At the same time, ammonium salts also cause emulsification between the distillate and water, which is highly detrimental to the defenolization process of naphthalene oil distillates. Therefore, tar must be desalted before distillation. Tar desalination involves adding an 8%~12% sodium carbonate solution to the tar before final dehydration, thereby converting the fixed ammonium salts into stable sodium salts. For example: 2NH4Cl + Na2CO3 === 2NH3 + CO2 + 2NaCl + H2O. These sodium salts remain entirely in the asphalt during tar distillation and turn into ash; if 0.1 g/kg of fixed ammonium salts is removed from the tar, the ash content of the asphalt increases by approximately 0.08%. Therefore, the amount of sodium carbonate added must be appropriate; when selecting the process route, it is necessary to ensure that the tar contains little water before final dehydration, which in turn results in a lower content of fixed ammonium salts (as fixed ammonium salts are soluble in water but not in tar).