sulphate process green liquor 1 is formed by dissolving the melt from the combustion of black liquor after concentration in water and thin liquor is transferred into a slaker 2 in which slaking and causticization are carried out during addition of quicklime the resulting precipitate deposited in the screening part is carried out of the slaker by means of a warm conveyor whereas the content of the slaker of milk of lime passes to causticizers 3 4 and 5 through overruns from the last causticizer the white liquor is carried together with calcareous silt to a separating and washing filter 6 from this filter the filtrated white liquor continues to a storage tank not shown the washed calcareous silt is transferred into a vessel 8 and further to a dehydration filter 9 subsequently the calcareous silt is carried to a rotary kiln for burning whereby supplementing lime not burnt can be added immediately before the rotary kiln at 14 the quicklime which can be supplemented with quicklime 13 is transferred to a lime silo 11 a conveyor mechanism 12 for the lime being situated below said silo this conveyor mechanism 12 carries the quicklime into the slaker 12 the process variables mentioned in the following example are for instance desired to be determined on two places in the causticizing plant place a and place b referred to as 15 and 16 respectively a flowmeter 17 a densimeter 18 measuring the tta.