| The ASAM Process for Sulfur free Pulping | Germany | - | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 46
Background
This case study was submitted on the part of the Working Group on Cleaner Production in Pulp and Paper Industries in the framework of the UNEP IE/PAC Cleaner Production Program with the support of the Technical Research Center of Finland's Non-Waste Technology Research Unit.
Cleaner Production Principle
New technology; Process modification
Cleaner Production Application
The alkaline sulfite anthraquinone methanol (ASAM) pulping process consists of three main sections. These are digestion and brownstock washing, screening and 4-stage bleaching, evaporation and methanol rectification.
The chips are pre-steamed for 15 minutes before the cooking liquor is added. A typical liquor consists of up to 25% inorganics, calculated as NaOH, based on dry wood, where 20% are Na2SO3 and the remaining 5% are added as NaOH or Na2CO3 or a combination thereof, 10-20% methanol, and around 0,1% of anthraquinone (AQ) as a catalyst. Cooking temperatures of 170-180oC must be maintained for 90-180 minutes. The cooking process can be modified to make different pulp grades for papers and boards by changing the cooking liquor's composition and pulping conditions.
Because unbleached ASAM pulp has very low lignin content, almost no extractives, and considerable initial-brightness, chlorine free bleaching is possible. After an initial oxygen delignification, ozone is used in a specially built reactor. Ozone has to be generated and consumed on site. Hardwood and softwood pulps can be bleached to approximately 90% ISO brightness with the sequence OZWP.
The filtrates from the bleaching stage are led countercurrent and are mixed with the black liquor after brownstock washing. This enables the dissolved organic substances and the inorganic bleaching chemicals to be caught in the black liquor.
The chemical recovery process stages are comparable to those used with sodium based sulfite pulping, after the methanol has been striped off the waste liquor evaporation and combustion, inorganic chemical conversion, cooking liquor preparation, and by-product making. Methanol separation and chemical conversion are added as different process steps to the recovery compared with kraft pulping. Most methanol can be condensed form the relief gases when the digester is opened, and the remaining part is stripped from the black liquor.
The AQ is almost completely dissipated in the cook, and none will be reclaimed.
As much water as possible is evaporated from the black liquor. the strong black liquor is burned in a boiler under reducing conditions. In the chemical recovery furnace, inorganic salts are recovered from the spent liquor in the smelt as sodium sulfide and sodium carbonate, and the energy values of the organic materials are recovered to make steam and electricity for the pulp mill. In the chemical conversion the inorganic chemicals Na2CO3 and Na2SO3 are regenerated.
The pilot plant capacities are:
| digestion and brownstock washing, assuming two cooking cycles, 1,3-1,5 moisture free metric tons/day |
| screening and bleaching 4,6-5,0 moisture free metric tons/day |
| water evaporation 29,0 metric tons/day |
Environmental and Economic Benefits
In regard to green field mill, investment cost for an ASAM mill are about the same as for a kraft mill. By July 1991 Kraftanlagen Heidelberg will have spent DEM 20 million for R&D, with 30% of the amount paid by the FRG's Ministry of Research and Technology.
Due to higher yields in ASAM cooks, savings in wood cost in case of hardwood of about 6-10% and in case of softwood of about 4-6% are gained. Cost for make up of cooking chemicals (Na2SO4, NaOH) are slightly higher for ASAM; reclaim efficiency of methanol is about 99%. Under certain conditions, caustizing plant and lime kiln are not required.
According to our calculation, an OZEP respective an OEP sequence results in lower operating cost than an OCDED or an C/D E/O DED sequence. Actual costs for ozonization largely depend on prices for chemicals and electrical energy, which vary from country to country and from mill to mill. Ozone consumption usually ranges from 5 to 8 kg/ton of pulp; production cost for ozone are estimated to be: in Finland (February 1991) US$ 1,28/kg, in Austria (June 1991) less than US$ 1,75 kg/ and in Germany (June 1991) US$ 1,45/kg.
Savings in effluent treatment are gained due to reduced flows.
An ASAM mill is as self-supporting in both heat and power as a modern kraft mill.
Payback time is as for a kraft mill.
No reduced sulfur compounds are made during the ASAM process and methanol is kept in a closed cycle system.
ASAM pulp can be bleached to high brightness in chlorine free sequences, and as a result there are no toxic, difficult-to-degrade organic chlorine compounds. The bleached pulp is also free of dioxins and furans.
Poisonous and oxygen consuming substances are nearly at a zero level in the effluents. Official wastewater quality requirements as stated in Germany can be met easily, including the limits for COD and BOD, and AOX will be zero because no chlorine chemicals are used, and for toxicity to maritime life.
High yield and superior quality are among the economies the ASAM process offers in comparison with other pulping processes. It also permits the use of some unbleached hardwoods for the production of writing and printing papers.
The chemicals in ASAM process can be recovered together because the same base is used in the bleach plant as in the cook. The process can be used with a wide variety of raw materials, and there is no special requirements for debarking or chip quality.
Constraints
Due to volatility of pulping chemicals no leaks and spills can be tolerated, both for environmental and economic reasons.
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.