Pulp & Paper II
Rev. March 1998
It was in June of 1994 that Vincent was invited to their first on-site testing in a paper mill. This has led to a major change in the Company: today more than half the presses being built are going to the pulp and paper industry.
Paper mills fall into three main categories:
- The virgin fiber mills where digesters cook wood chips to make paper pulp
- Recycle mills where waste paper is pulped by agitating it in a water bath
- Mills that buy market pulp from other mills and convert it into their own specialty papers.
Vincent has screw presses operating in all three of these industry segments. In most cases our press is being used to dewater waste streams.
The waste streams generally have consistencies in the range of 1% to 12% solids. They are dewatered so that the solids become a bulky material that can be readily handled. At the same time the pressate liquid frequently becomes clear enough that it is acceptable for re-use in the mill.
The waste streams that can be dewatered in a Vincent press consist of various forms of screen rejects. In a virgin fiber mill rejects come both before and after bleaching. Ahead of bleaching we deal with brown stock rejects: knots from the knotters, and shives (along with a fair percentage of good fiber) from the tertiary or quatenary pressure screens.
In a recycle mill a variety of screenings are used to separate unusable fiber along with dirt, ink, plastic, etc. All of the waste streams will contain some measure of good fiber.
The press cake will typically be in the range of 40% to 50% solids. The lower solids content is adequate for easy handling and transport, while the higher solids material might be used for boiler fuel. (In the boiler fuel application the press cake is combined with 80% to 90% coal.) Frequently the press cake is landfilled. However occasionally the press cake is sold to mills capable of using the material in the production of lower grade pulp products. Other uses include filler for tar paper and shingles as well as landscaping mulch.
This is where we find the market.
The most common inquiry from the pulp and paper industry involves pressing clarifier sludge at the wastewater treatment plant. We have found that our press will usually work well on this material. This is especially true if there is little or no biological (secondary treatment) sludge present. Our competitors in this market are the immense continuous screw presses offered by Andritz and FKC.
With our press design we have found that the best performance is achieved if the clarifier sludge is prethickened with a gravity table or a belt press. To address this opportunity we have established a relationship with Phoenix Process Equipment Company, a major supplier of belt presses to the pulp and paper industry.
Future issues of Pressing News will detail our applications in both virgin fiber and recycle mills.