Throughout this application, it is to be understood that the term peroxide is intended to mean the total of all peroxide in the solution including the ionized fraction and the un-ionized fraction, i.e. the salts of peroxide HO.sub.2.sup.- ions, which generally form the ionized fraction and the H.sub.2 O.sub.2 which forms the un-ionized or neutral fraction.
The use of membranes to separate materials from water by reverse osmosis is a well known technology. Many different membranes have been devised generally each for a very specific purpose in mind, i.e. to invoke a specific separation of selected material from water or other liquids to increase the concentration of the selected material. In the processes of which applicant is aware, the feed liquor is fed to a membrane which separates the feed liquor into a retentate and a permeate using a membrane specifically designed to obtain the desired type of separation. Generally this process is used to produce water of higher purity from water containing contaminants, the retentate contains the contaminants at the higher concentration than the incoming water and the permeate provides the purified water.
In the process of producing hydrogen peroxide, the hydrogen peroxide is generally produced as an aqueous solution containing both peroxide and anthraquinone. The anthraquinone is separated by means of an organic solvent to produce a low concentration solution of peroxide in water. The concentration of hydrogen peroxide in the solution is increased by evaporating water from the solution.
In the production of pulp from cellulosic material, for example in the manufacture of paper, hydrogen peroxide is used in various ways. It is most commonly used for bleaching mechanical pulp, for example stone groundwood, thermo-mechanical pulp (TMP), or chemi-thermo-mechanical pulp (CTMP). It is also known to use hydrogen peroxide in the bleaching of so-called chemical pulps namely pulp from the kraft or sulfite processes or other chemical pulps. A common means of employing hydrogen peroxide in this case is in the caustic extraction stage. It is also known to use peroxide, particularly alkaline peroxide to treat wood chips in refining processes to produce a mechanical pulp as well as in de-inking and recovery of secondary fibre, for example from old newspapers.
In the bleaching of chemical pulp with hydrogen peroxide, no attempt is made to recover residual peroxide. Similarly in the mechanical pulping processes dilute residual peroxide may be recirculated although it is not known to purify or concentrate this stream.
In the bleaching of mechanical pulp using hydrogen peroxide the bleaching is normally carried out at a consistency (solids content) of 3-40%, most usually 12 to 30%. At the end of the brightening reaction, a considerable portion of the original charge of peroxide is normally present in the liquid phase. A residual peroxide containing liquid stream can be separated from the pulp using a thickening device such as a press. Some attempts have been made to reuse the residual peroxide by recirculating this stream to an earlier stage in the brightening process or by using it to brighten a second portion of the pulp. These processes are known by the applicant to encounter problems for at least two reasons: (1) In the bleaching of pulp with hydrogen peroxide, undesirable materials are generated in the liquid phase by the brightening reactions. In a recycle process using dilute residual peroxide, these undesirable material build up, and adversely affect the bleaching reaction efficiency. (2) Because of the dilution and rethickening stage required to recover a solution of residual hydrogen peroxide for reuse, the concentration of recovered hydrogen peroxide in these processes will be low. This has the disadvantage of liquid and stock handling problems due to the low consistency (solids content) in the process where the residual peroxide is reused.