flows in the direction of arrow a through a pipe 22 which carries the heating medium to the inlet 20 as shown in the drawing the heating medium is fed into the heater vessel 12 downstream of heating section 14 preferably being located below the lowermost baffle 18 the gaseous heating medium then flows upwardly through the heating vessel 12 around baffles 18 and exits the vessel 12 in the direction of arrow b through a vent pipe 24 a first portion of the slurry enters the heater vessel 12 through a first slurry inlet 26 the first portion of the slurry is then pumped through a perforated pipe 28 within the heater vessel 12 and is sprayed from the perforations in the pipe into the interior of the heater vessel 12 the first portion of the slurry then passes downwardly by gravity through the heating section 14 of vessel 12 passing down across the surfaces of the baffles 18 in the direction of the solid arrows shown inside the vessel 12 as the first portion of the slurry passes downwardly through heater section 14 it is brought into contact with the heating medium rising upwardly through the vessel 12 thereby increasing the temperature of the slurry the first portion of the slurry heated as described above is collected in collection section 16 the approximate level of the collected slurry inside section 16 is designated as numeral 30 a slurry outlet 32 is provided adjacent the slurry collection section 16 and is preferably provided at the lower.