Classifying – separating – and assorting solids – Fluid suspension – Liquid
Patent
1999-05-06
1999-12-14
Lithgow, Thomas M.
Classifying, separating, and assorting solids
Fluid suspension
Liquid
209170, 162 4, B03D 124, D21C 502
Patent
active
060005521
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for separating substances from a fiber suspension and more particularly to such a process wherein impurities, such as for example printing inks and other contaminating substances, are separated from the fibers in a flotation tank into which the fiber suspension in admixture with gas is introduced by means of directed jets having a high inlet velocity.
Flotation is a separation process which on an industrial scale primarily is utilized in mineral treatment and for purification of recycled fibers in order to separate undesirable components by treatment of a liquid based system of solid materials. Flotation processes can be carried out using methods wherein the essential separation force is either gravitation or centripetal acceleration. The flotation process is based on introduction of a gas, usually air, to which hydrophobic particles are attracted and subsequently can be separated in a foam phase. Different auxiliary chemicals are often used, especially surface active agents, for example in order to improve the hydrophobicity of the particles to be separated, to enhance foam formation etc.
In the pulp and paper field, treatment of recycled fiber by processes for removal of ink, often combined with bleaching, for production of recycled fiber pulp with a desired brightness, is a more and more growing area. The raw material, the recycled paper, is beaten with a high intensity while being dissolved to a consistency which usually is around about 15 percent by weight. Internal friction in the suspension due to high consistency causes strong shearing forces in the suspension which, when the fibers are rubbed against each other, results in release of ink and other impurities. In this preliminary step additions of certain chemicals can be made, for example addition of surface active chemicals for the subsequent flotation process and/or of bleaching chemicals such as hydrogen peroxide for later bleaching steps. After the initial slushing step there normally follows one or several screening steps in order to remove larger particles before the suspension is prepared for the actual removal of the printing ink. The suspension is hereby normally diluted to a consistency of about 1 percent by weight. Although addition of chemicals usually is made at the slushing, it is also possible to add auxiliary chemicals for the flotation at the dilution step. At the actual flotation the fiber suspension and air are introduced separately or in admixture into a flotation tank or cell and separation of the ink particles occurs as the particles adhere to the air bubbles and can be separated in the form of a foam. Subsequent treatment usually comprises certain additional separation steps and dewatering, dispersing, bleaching and dilution. The flotation treatment and optional subsequent steps are often repeated once or several times. To reduce fiber losses in the flotation the separated foam is usually treated in a secondary flotation step.
In contrast to flotation processes in the mineral industry there are special problems connected with achieving a good separation at flotation treatment of fiber suspensions, since the fibers form networks which obstruct the transport of the gas bubbles through the suspension. Due to this it has been necessary to work with very low fiber consistencies in the suspensions, which means that extremely large amounts of water have to be handled and this is, of course, a great disadvantage both with regard to economy and environment. Within the field of flotation of fiber suspensions an increase of the fiber concentration in the suspension which is small in terms of percentage thus involves an extremely substantial reduction of the total amount of water and a reduction of the flotation time is likewise an extremely important advantage.
One object of the present invention is to make treatment at increased fiber concentrations possible in purification of fiber suspensions, particularly in deinking of recycled fiber. The process of the invention makes this possible with
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Eriksson Thomas
Fredriksson Borje
Wikstrom Tomas
Lithgow Thomas M.
Sunds Defibrator Industries AB
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