Treatment of Pulp

Abstract

A method of treating a fibre pulp mixture comprises: (a) separating the fibre pulp mixture into at least two fractions to form a coarse fraction and a fine fraction; (b) bleaching the coarse fraction from (a); and (c) precipitating an alkaline earth metal carbonate in the fine fraction from (a). The products from steps (b) and (c) may combined in an integrated paper making process.

Description

FIELD OF THE INVENTION[0001]The present invention relates to methods for treating pulp for use in the production of paper and paper products, in particular super calendered (SC) paper. The present invention also relates to products obtained by said methods.[0002]The term “paper” should be understood to mean all forms of paper, including board, card, paperboard, and the like.BACKGROUND[0003]One of the most important characteristics of paper is its degree of whiteness. Generally, and depending on the application, the whiter the paper, the higher the selling price. In order to obtain whiter papers, efforts have been directed to treatment of pulp by subjecting it to bleaching agents such as alkaline hypochlorite.[0004]There are numerous disadvantages associated with bleaching pulp in order to increase the whiteness of paper. For example, in bleaching pulp, large quantities of bleaching agent are required. This contributes to increased production cost, as well as being an inefficient use...

AI Technical Summary

Benefits of technology

[0019]In a further aspect of the present invention some or all of the mixed aggregated material resulting from step (c) may be combined with the bleached coarse fraction resulting from step (b) according to the first aspect of the invention, in order to make paper. This aspect of the invention advantageously results in minimising the various waste streams and helps to maintain the balance of the plant in which the paper making process is being carried out.

[0020]There are numerous other advantages associated with the present invention. Although bleaching may be performed in the normal manner, the bleaching equipment may be smaller in size compared to conventional bleaching units, due to the smaller mass to be bleached. In addition, as the coarse fraction will generally dewater more efficiently than the original pulp, the devices for carrying out the dewatering may also be reduced in size. In general, the bleaching chemical consumption for a given weight of material is proportional to the surface area of the material. The removed fine fraction will in general have a higher specific surface area than the coarse fraction and consequently the bleaching chemical consumption for the coarse fraction is lower for a unit of weight than that for the original pulp; it is estimated by about 10-15%. The investment costs for a bleaching plant according to the process of the present invention may be significantly less than the costs for the current type of plant; typically up to about 30% less. It is estimated that the running costs, mainly due to the decreased use of resources, for example, chemicals and electricity would be about 15% lower than for a conventional plant.DETAILED DESCRIPTION OF THE INVENTIONThe Fibre Pulp Mixture

[0029]In the method described in US 2004 / 0011483, a mechanically manufactured fibre pulp mixture containing fibres of varying lengths is screened into at least two fractions containing fibres of mainly varying lengths. The invention described in US 2004 / 0011483 allows for the accurate separation of a fibre fraction that is shorter than a fraction representing a particular length from the fibre pulp mixture. In particular, the apparatus described in US 2004 / 0011483 comprises a gap screen to separate short fibres comprising a convergent gap and at least one wire restricting the convergent gap. The fibre pulp mixture to be screened is fed into the convergent gap so that it flows through the gap screen, in the same direction as the wire, towards the convergent end of the gap so that the shorter fibres and some water exit through the openings in the wire and the remaining part of the fibre pulp mixture exits from the gap screen through the output port at the convergent end of the gap.

[0032]After separating the coarse and fine fractions, the coarse fraction is bleached in a standard manner. Prior to bleaching, the coarse fraction may be dewatered and optionally washed using any of the known standard procedures, for example, by use of a belt press. Dewatering of the coarse fraction may advantageously give rise to better economy and efficiency of the overall process. Depending, for example, on the type of solids present, dewatering of the coarse fraction typically results in the solids content in the coarse fraction increasing from a range of about 0.01 to 0.1 wt % to a range of about 1.5 to 6.5 wt %. Due to the reduced mass of the original pulp mixture, the equipment needed for the bleaching process may advantageously be smaller than standard bleaching equipment. Bleaching processes are well known to those skilled in the art and those processes suitable for use in the present invention will be readily evident. For example a reducing bleaching agent may be used such as a dithionite salt, for example sodium or zinc dithionite. Other examples of reducing bleaching agent include zinc dust, thiourea dioxide (i.e. formamidine sulphinic acid) and sulphur dioxide. Oxidising bleaching agents may also be used and hydrogen peroxide is a particularly preferred bleaching agent. Other suitable oxidising bleaching agents include peracetic acid and ozone. The amount of the reducing bleaching agent used is preferably in the range of 1.5 g to 7.5 g of the reducing bleaching agent per kilogram of dry fractionated pulp material and in the case of oxidising bleaching agents the corresponding amount is preferably in the range of 1.0 g to 4.0 g of the oxidising agent.The Fine Fraction

[0035]Dewatering of the fine fraction may advantageously result in a better quality carbonation product and provides cleaned water for recycling in the process of the present invention. In the present invention, the solids content of the fine fraction is generally increased from about 0.03 wt % to 6.5 wt %, more particularly to a range of about 0.2 wt % to about 4 wt %, for example to about 1.5 wt %; in particular this increase may be achieved with the use of DAF units. The use of a combination of dewatering units, such as DAF units, may advantageously lead to the use of units of reduced size and / or the amount of waste solids in the overall process being reduced.

[0048]Calendering is a well known process in which paper smoothness and gloss is improved and bulk is reduced by passing a coated paper sheet between calender nips or rollers one or more times. The methods according to the present invention are particularly suited for the production of super calendered paper.

Problems solved by technology

There are numerous disadvantages associated with bleaching pulp in order to increase the whiteness of paper.

For example, in bleaching pulp, large quantities of bleaching agent are required.

This contributes to increased production cost, as well as being an inefficient use of large amounts of bleaching agents.

Balancing these aspects has hitherto proven difficult.

One of the other environmental issues relating to the production of paper concerns the aqueous waste streams produced by paper mills, which generally comprise suspensions of fine organic microfibres, for example cellulosic fibres which typically have a length of no greater than about 75 μm, and other organic materials usually in association with inorganic particulate materials.

The solids in such streams have, in some cases, proven difficult to dewater; in addition the waste streams are environmentally and economically undesirable to discharge.

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