| Direct Alkali Regeneration System (DARS) in Small Pulp Mills | India | 1986 | Pilot scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 50
Background
The Mandya National Paper Mills in India practices the soda pulping of bagasse. In early 80's, Central Pulp & Paper Research Institute (CPPRI), India, worked on ferrite auto-caustizing process and it was shown that the process has a potential for application in small mills. Extensive studies conducted in Australia led to the development of the process on a commercial scale and a 200 tpd recovery unit, based on this process was commissioned in early 1986. CPPRI continued efforts to further develop this process to suit silica rich spent liquors.
Cleaner Production Principle
New technology; Process modification
Cleaner Production Application
The basic principle of the DARS process is involving production of sodium ferrite by auto-causticization of sodium carbonate with ferric oxide and subsequently to obtain sodium hydroxide by hydrolysing the sodium ferrite.
Silica is an undesirable element in all the spent liquors from pulping of agricultural residues. Extensive studies on the effect of presence of silica impurity in the recovery loop revealed that during ferrite auto-causticization process only a minor proportion of silica passes into white liquor. This is an advantage of the process and unlike conventional recovery it may not be necessary to go in for an additional stage of desilication of spent liquor prior to the recovery operation.
A substantial quantity of natural haematite ore as a source of Fe2O3 for auto-caustizing will be used in the ferrite process. There is a likelihood of the presence of some soluble iron compounds in the ore which are subsequently carried to the white liquor regenerated in the ferrite process. Studies on the solubility of iron reveal that when high purity haematite ore is used the solubility of iron is negligible and does not have any adverse effect.
Leaching is a key step of the ferrite process and conditions maintained should allow maximum causticity and soda recovery. It is necessary to ensure that the sodium ferrite is completely hydrolyzed and that maximum extraction of sodium hydroxide in the form of concentrated solution is achieved. The most efficient leaching configuration will be pure counter-current contacting. Results show that a minimum of four stages of extraction would be required for a sodium recovery over 90%.
The arrangements for ferrite studies at the mill were as following: The capacity of the roaster was about 1,65 tons of black liquor solids/hour at 50% concentration. At 40% the capacity was about 1 t/h. The retention time of black liquor in the roaster was about 15-20 minutes. The thick black liquor had a specific gravity of about 45 Tw. About 0,5 m3 of this black liquor was drawn into the mixing tank to which haematite ore was added and mixing was effected using an agitator. The resulting mixture was pumped into one end of the roaster and furnace oil was fired from the other end of the roaster. During burning the flow of the furnace oil is maintained to get the desired temperature in the roaster.
It should be noted that this is in the pilot stage of development.
A commercial scale recovery unit of 200 tpd capacity is working in Australia.
The technology is commercially available.
Environmental and Economic Benefits
The DARS process has the following benefits: the process has operational flexibility, it is compact and simple in operation and also requires less space. The process permits the most economic use of fuel by way of minimized quantity of high cost fuel, high degree of process automation is not required. Unlike the smelt in conventional recovery the combustion product is solid, so the process is safe. The capital cost of the DARS process is low.
Constraints
From the activity of regenerated Fe2O3 it is felt that ore should not be recycled more than six times and the reduced activity of silica rich ore was attributed to brittleness.
The causticity varied from 2-73%. It was observed that the reasons for low level of causticity were: It was not possible to monitor the temperature within the roaster; There was no sufficient draft to expel the CO2 from the roaster. The increased partial pressure of CO2 resulted in poor decomposition of sodium carbonate and subsequently low causticity values. The quality of the ore used was very coarse and contained lower proportion of Fe2O3; The retention time inside the roaster was not uniform.
Contacts
Review Status
This case study was submitted by the UNEP Working Group on Cleaner Production in the Pulp and Paper Industries, based at the Technical Research Center of Finland (address above) in 1992, as part of a contract for UNEP IE. Before submission the case studies were reviewed at the Center. They were edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.