Chemical Recovery Cycles
The closure of the cycles in an alkaline pulp mill requires increasing data from the complex chemistry of the recovery area. Of particular interest is the speciation chemistry of the green liquor and the lime cycle, as here the formation of dregs and mud precipitates provides a feasible option for the removal of undesired compounds from the process. In this work, the co-precipitation of MoO42- with calcite was studied experimentally and with a multi-component thermochemical simulation model. Thermochemical modelling was further applied to simulate lime kiln reactions.
Figure 1. Recaustization cycles provide feasible options to remove undesired compounds from the closed pulp mill.
Soluble Na2MoO4 may be used as catalyst in acidic peroxide delignification. The Mo-content can then be carried over with black liquor to recovery boiler and to soda ash. Part of the Mo is reduced to insoluble forms and becomes removed with dregs. The soluble part remains in green liquor as Na2MoO4. In caustization, the following reactions are important:
where a denotes aqueous and precipitating species.
Experimental: The solubility of Mo during caustization was tested with the concentration levels of 50 and 1000 ppm. The Na2MO4-solution was added to green liquor which was then causticized with CaO. The solution was clarified and sentrifuged. From the solution, caustization efficiency and soluble Mo were determined.
Thermochemical model: The causticizing reactions I & II were also simulated with a multi-component thermodynamic model with standard thermodynamic input data. The introduction of Ca(OH)2 in the model was done stepwise with an initial image component so as to simulate the gradual increase of caustization efficiency towards equilibrium. The deposited species were modelled as co-precipitated solid solution or as pure substances.
The result of the study is shown in figure 2. Both reactions are thermodynamically viable, but reaction I dominates while aqueous Na2CO3 is present. A combined reaction kinetic and thermodynamic model was further applied to study the reaction conditions in the slaker-caustizicer system and in the lime kiln. The recovery of aqueous sodium molybdate [ Na2MoO4(a)] from the green liquor was found to be difficult in low-to-moderate caustization efficiencies. As little or no co-precipitation of MoO42-(a) takes place in the caustization reaction, a high caustization efficiency is required to recover the molybdate ion as CaMoO4(s) deposit. Thus the caustization reaction becomes also the rate limiting factor for the precipitation. Practical process experience was found supportive to the modelling and laboratory data. The molybdate throughput yet remains slight and has little or no effect in cooking.
Figure 2. The Mo-content of green liquor during causticizing: caustization model curves vs. measured points
Figure 3. The thermodynamic image modelling was also applied to combined heat transfer and reaction profiles of the lime kiln.
The recaustization and lime cycles can be modelled with multicomponent thermochemical methods which include the salient chemical reaction and mass transfer rates2. When supported by experiments and process data, the models can be used to optimize recovery operations.