Today's Trends in Coating Kitchens, Supply Systems and Coating Effluent Treatment

摘要

Basic coating colour preparation systems have remained quite similar in design concept for many years. The demand for proven reliability, together with characteristic paper industry conservatism has prevented overly "radical" solutions from entering the market. However there are still a number of new processes and pieces of equipment design which are slowly becoming industrial standards. This paper presents some selected developments and new products which should prove to be of common interest among paper makers of various grade ranges. Pigment dispersion and coating colour mixing is known to have vital impact upon final coating mix properties. The pigment slurry is the largest single component of the coating colour formulation, and is also responsible for introducing the majority of the impurities. Inadequate processing can cause defects which are difficult to correct in subsequent process steps. Ever increasing solids contents and viscosity levels of the coating mix demand a particular capability of the mixer in use. In fact there is a lot of equipment operating in paper mills today which simply offers inadequate shear and mixing properties. As an alternative to installing a complete new mixer, however, there are available agitator retrofit packages which can improve mix preparation performance, without major capital investment. Continuous coating colour preparation is becoming increasingly popular. It offers benefits especially in processes where few coating colour grades are required and particularly where formulations are quite similar to each other. The advantages which may be realised relate to investments costs, space demands, fast grade changes and minimised colour losses. Batch processing, however, is still the most common technology in use today, and will likely remain so for the foreseeable future. There have been developments, mainly in dosing systems, recipe handling and automation. There are also novel dosing systems which make the usage of dry components a reality. The majority of problems in the supply system are related to screening. There is a good choice of available technologies which better take into account the special features of each application, such as multistage screening, automated strainers, return line pressure screening, in addition to the development of a new screening element having round holes. Centrifugal deaeration is today accepted as essential for use in conjunction with some coating technologies, but offers benefits, in fact to any method. On line measurement of colour properties such as solid content, air content, rheology, pH and temperature is today commonplace. Coating methods have also been developed which allow complete elimination of return flow, which permits design of a very compact supply system, saving space and start up time and allowing much simplified clean up and servicing procedures. Ultrafiltration is now one of the principal methods for coating effluent treatment, and is unique as a method which enables recycling of pigments and chemicals with retention of their full functional value. That is to say that the effluent concentrate can be used as a component of the new coating colour. In some applications, the limiting factor has been the concentrate solids at 25 - 35 % level. Adding a small second stage filter, however, can see this increase to 50% level, which allows higher dosing amount into the new coating.