The brown coal liquefaction (BCL) process is a two-stage liquefaction (hydrogenation) process developed for Victorian brown coal in Australia. The BCL process has a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product (vacuum residue) derived from the coal in primary hydrogenation and named CLB (coal liquid bottom). This solvent de-ashing step uses toluene or coal-derived naphtha as a de-ashing solvent (DAS). After dissolving the CLB into the solvent (CLB/solvent ratio, 1/8-1/4, w/w) under high temperature (200-290 degrees C) and high pressure (4-5 MPa), insoluble solid particles which consist of ash and heavy preasphaltenes are settled by gravity and separated from the solution as an ash-concentrated slurry. The ash-concentrated slurry and the de-ashed solution are withdrawn from the settler as an underflow and overflow, respectively. The de-ashed heavy product is recovered from the solution by eliminating the solvent and is further hydrogenated in secondary hydrogenation. The authors have reported on the solubility of CLB in toluene and the settling velocity (V) of the boundary of ash content in the settler under de-ashing conditions. This paper discusses the effects of de-ashing conditions on ash concentration in the settler bottom and the operating conditions of a continuous de-ashing system. The ash content in underflow (C-UF, kg/kg or wt.%) at the settler bottom was found to increase with temperature and to decrease with the rate (flux) of downward flow (underflow). The maximum C-UF, Z, is expressed by the equation: Z = B-CLB(FL/0.35)(-0.32)(T/527)(4.26), where B-CLB, FL and T are the characteristic parameters of organic CLB (kg/kg or wt.%), flux of underflow in the settler (kg/m(2) s) and temperature (K), respectively. B-CLB is also expressed by using the analytical results of organic insolubles in the CLB under de-ashing conditions. Finally, stable operating conditions of a continuous de-ashing system are confirmed to be determined as the following qualifications: /V-u/ < /V/, W-UF > W-SA/C-UF and Z > C-UF, where /V-u/, /V/, W-SA and W-UF are the upward velocity of the solution in the settler (mm/s), settling velocity of the ash boundary (mm/s) in the settler, flow rate of ash in the feed slurry (kg/h) and flow rate of underflow (kg/h). respectively. Under these qualified conditions, the 50 t/d pilot plant constructed in Australia was operated under stable conditions for 3700 h using toluene as a DAS.