Method and apparatus for separating impurities from machine circulation of coating station

A method for separating impurities from a machine circulation of a flow of processing agent used in a processing station to coat a paper or board web, the method comprising the steps of:

• leading the flow of the processing agent from a reservoir at a first level with a pump through a gap sieve to the processing station at a second level;
• spreading at least a portion of the flow of processing agent on the surface of a paper or board web, coating the web, and producing an excess flow of processing agent which leaves the processing station;
• and
• leading at least part of the excess flow of processing agent to at least one perforated drum pressure sieve using a perforated plate as a separating element at a third level between the first and second levels, wherein a flow of processing agent coming from the at least one perforated drum pressure sieve is led at least partly to at least one reservoir, which is located below the level of the perforated-drum pressure sieve in such a way that a feed pressure of the processing agent through the perforated drum pressure sieve is created entirely by the difference in height between the at least one reservoir and the at least one perforated-drum pressure sieve.

An arrangement in a paper or board machine for separating impurities from a machine circulation of a processing agent mixture in a coating or sizing station comprising:

• a processing agent mixture reservoir positioned at a first level;
• a pump in processing agent mixture receiving relation to the reservoir;
• a gap-drum pressure sieve in processing agent mixture receiving relation to the pump;
• a processing station positioned at a second level, in processing agent mixture receiving relation to the gap-drum pressure sieve, the processing station engaged in processing agent mixture spreading relation with a paper or board web, the processing station having a collector pan arranged to receive processing agent mixture;
• and
• at least one perforated-drum pressure sieve using a perforated plate as a separating element located at a third level between the second level and the first level, the perforated-drum pressure sieve positioned at a height difference below the level of the collector pan and connected in processing agent mixture receiving relation to the collector pan by a return pipe which connects to a branch line which leads to the at least one perforated-drum pressure sieve through a first valve, and to the processing agent mixture reservoir through a second valve, such that the height difference between the collector pan and the perforated-drum pressure sieve is arranged to create a feed pressure in the processing agent mixture supplied to the perforated-drum pressure sieve, wherein the at least one perforated-drum pressure sieve is in processing agent mixture supplying relation to the reservoir through a third valve;
• and
• a pressure sensor in the return pipe and a regulator connected to the sensor and the third valve in pressure regulating relation so that the pressure in the perforated-drum pressure sieve and the return pipe can be regulated so the perforated-drum pressure sieve and the return pipe are filled with processing agent mixture.

A method of separating impurities from processing agent mixture recirculated from a sizing or coating station, the method comprising the steps of:

• drawing coating from a reservoir with a pump and pumping the coating through a mechanically cleaned sieve having a sieving element in a closed pressurized vessel in which the coating travels through the sieving element from a higher pressure to a lower pressure, and passing the coating through a linear gap of about 75 μm to 150 μm defined between a triangular wire wound in a cylinder to a processing station and applying the coating to a board or paper web;
• collecting coating which is doctored off the paper or board web in a collecting pan, and leading at least a portion of said collected coating through a return line to at least one perforated drum pressure sieve, located below the processing station, and separating fibers from said collected coating with a perforated plate forming a part of the perforated drum pressure sieve;
• passing an accept fraction flow of the collected coating through a regulating valve to the reservoir;
• and
• regulating the accept fraction flow with a regulator based on the output of a pressure sensor in the return line before the at least one perforated drum pressure sieve in such a way that the sieve and the return pipe are filled.

A method of coating a paper web and controlling and processing a flow of processing agent, the method comprising the steps of:

• pumping a flow of the processing agent with a pump from a machine reservoir at a first level to a cylinder wound from a single triangular wire, in which a gap for penetration is formed between the wire windings forming a gap sieve, and removing impurities from the processing agent in the gap sieve;
• following the gap sieve supplying the processing agent to a coating station at a second level above the first level, and coating the paper web by spreading a portion of the flow of processing agent on a surface of the paper or board web, coating the web and producing an excess flow of processing agent;
• collecting the excess flow of processing agent in a collector pan and leading the excess flow of processing agent by a return pipe to a branch line which leads to at least one perforated-drum sieve using a perforated plate as a separating element, through a first valve, and to the machine reservoir through a second valve, wherein the perforated-drum sieve is located on a third level between the second level and the first level;
• wherein the first level, the second level, and the third level are arranged to create a pressure differential over the perforated-drum sieve sufficient to cause processing agent to pass through the perforated-drum sieve;
• filtering the excess flow of processing agent through the perforated-plate of the perforated-drum sieve, the perforated-drum sieve having an accept side receiving processing agent which has passed through the perforated-plate, the accept side connected to the machine reservoir through a third valve;
• and
• measuring a pressure in the return pipe with a pressure sensor and regulating the third valve and thus the pressure in the perforated-plate sieve and the return pipe in such a way that the perforated-plate sieve and the return pipe are filled with processing agent.