WO1999000545A1

WO1999000545A1 - Method of reducing the content of calcium in the liquid phase of a pulp suspension

Info

Publication number: WO1999000545A1
Application number: PCT/SE1998/001053
Authority: WO
Inventors: Ulf GERMGÅRD; Per-Åke SÖDERSTEN
Original assignee: Sunds Defibrator Industries Ab
Priority date: 1997-06-30
Filing date: 1998-06-03
Publication date: 1999-01-07
Also published as: SE9702512D0; SE9702512L; SE511794C2

Abstract

The calcium content is reduced in the liquid phase of a suspension of paper pulp, which in a bleaching process has been subjected to a pH reduction to a pH under 7, partly by alkalizing the pulp, partly by treating the pulp with carbonate for adsorption of calcium compounds onto the pulp.

Description

METHOD OF REDUCING THE CONTENT OF CALCIUM IN THE LIQUID PHASE OF A PULP SUSPENSION

The present invention relates to a method for reducing the calcium content in the liquid phase for a suspension of paper pulp which has been subjected to a pH reduction in a bleaching step, and more specifically to a method wherein the calcium content in the liquid phase is reduced by partly subjecting the pulp suspension for a pH raise and partly by the addition of carbonate.

In the future, paper and pulp mills will be closed sub- stantially harder than today and this might lead to that the finished products will contain higher amounts of different trace substances, which today are washed out during the manufacturing process and thereby are carried out into the final receiver. Alternatively qualified mechanisms will be necessary for removing different trace substances, adapted to the different types of compounds. Already today there are hard- closed mills in which the needs of being able to handle these problems satisfactorily are great.

Calcium ions are often troublesome in pulp bleaching. Normally they enter the pulp mill together with the wood and with the cooking chemicals, and are bound hard to the paper pulp as long as this is in the traditionally closed part of the process, i.e. in the cooking part or in the oxygen step, as long as the pulp is not subjected to a pH reduction to under a pH of about 8-9. Normally, however, many of the subsequent bleaching steps are performed at a pH below 7, which means that calcium ions are released from the pulp and if the system is closed hard, an accumulation of calcium ions can arise in the filtrates from the process. This might lead to that many different calcium precipitates are formed. Precipitates of difficultly soluble calcium precipitates might lead to clogging of pipes and other equipment, reduced function of the equipment and soiling of the pulp. The total control and regulation of the process are thus risked by such precipitates. Therefore, it is very important to avoid precipitates and scale and efforts to this end has been directed to different methods of binding calcium, e.g. which sequestering agents in order to keep it dissolved, or to remove calcium from the flow e.g. by different types of precipitation methods. So called hard closed alkaline bleaching sequences have been studied in many places, when as is noted above, the calcium ions at a pH above about 8-9.are bound to the pulp and will follow it out from the mi1-1. However, completely alkaline bleaching sequences have not been as effective as traditional sequences with both alkaline and acid bleaching steps and there are reasons to question if it is even possible to achieve the same effectiveness with completely alkaline bleaching steps.

A theoretical possibility to solve the problem could be that the paper pulp after an acid bleaching step is re- alkalized to a pH above about 8 in order to readsorb the dissolved calcium ions onto the pulp and in that way achieve a natural expelling of these ions from the bleaching step. A method based on pH changes is described in WO 95/09944 which discloses that scale formation because of salts, primarily calcium oxalate, is reduced or eliminated by, for pulp having been subjected to a bleaching sequence with at least one alkaline step and at least one acid step, generating a filtrate flow containing dissolved salts and combining this or part thereof with alkaline material and pulp in order to have the salts combine with the pulp and be removed from the filtrate flow. Normally, however, it is not possible to readsorb all ions having been released by only increasing the pH, but instead often a substantial part of the ions remain in the solution also after a pH increase to a level substantially above 8. A possible explanation to this is that in the original pulp there are calcium ions, partly bound to the pulp in the form of electrostatic bonds, partly it is assumed that part of the ions are bound in the form of complexes . These complexes are not reformed after acidification. The present invention offers a solution to the above problem in that calcium more effectively can be transferred to and/or be bound to the pulp in a bleaching process which comprises steps at different pH values. The fact that calcium according to the present method more effectively and to a higher extent is transferred from the liquid phase to the pulp, fibres, and to a great extent is maintained therein in the form of calcium carbonate, reduces the risk for scaling and indirectly leads to a reduced environmental load, since the process can be made more closed. At the same time, no negative influence on the final product, i.e. the paper pulp or the final paper, is experienced, since calcium carbonate really is^" a conventional filler for paper and the small amounts obtained in the present method compared to the amounts used as filler, has thus no negative influence. Accordingly, the present invention relates to a method which is more precisely defined in the appended claims.

The invention will be explained more in detail below, with reference to the accompanying drawing, in which the graph shows the calcium content in the pulp versus amount of added Na₂C0₃.

According to the present invention the content of calcium in the liquid phase for a suspension of chemical paper pulp is reduced, which pulp is subjected to a bleaching method wherein the pulp suspension in one step has been subjected to a pH reduction to under pH 7 in that the suspension partly is alkalized to a pH above 8, partly carbonate ions are added so that calcium compounds, mainly in the form of calcium carbonate, are formed in the suspension and are adsorbed to the pulp and are therewith transferred with the pulp to the next step of the process.

The present method is especially applicable when a pulp suspension in one step has been subjected to a pH reduction to a pH underneath 6, such as in a bleaching step with peracetic acid, chlorine dioxide or ozone, especially under 4, and which subsequently shall be alkalized to a pH above 8, especially a pH of at least 10, for example to be subjected to a hydrogen peroxide step. In connection with the alkalizing step, the pulp suspension is treated according to the invention by addition of carbonate ions, which preferably can be made by addition of a carbonate-containing solution. An addition of carbon dioxide is also possible, but then a certain alkali addition is needed in order to avoid a reduced pH.

The alkalizing is made as is conventional, suitably with sodium hydroxide or, when it is possible, with oxidized white liquor. In connection therewith, simultaneously or thereafter, carbonate ions are added. This can be made by addition of a carbonate-containing solution such as e.g. a solution of sodium carbonate or by the addition of carbon dioxide. The addition should give at least 0.1 kg carbonate ions by the ton pulp, suitably from 0.1 to 10 kg and preferably 1 to 3 kg by the ton pulp. In the Figure, the Ca content (g/odt) is plotted versus added Na₂C0₃ (kg/odt) . The Figure illustrates that by- a carbonate addition at the same time as the pH is increased from acid to alcaline, the calcium content of the pulp is signifi- cantly increased. When no xx is added the calcium content in the pulp is about 1,100 g/odt, while an addition of 6 kg/odt gives a Ca content of about 2,200 g/odt.

The treatment with carbonate ions can be made in a very short time, from a couple of seconds, up to a couple of hours and the time period for the treatment lies suitably within the interval 0.1 to 120 minutes and preferably within the interval 0.1 to 15 minutes. The temperature of the treatment should exceed 40°C and preferably exceed 60°C, since the solubility for CaC0₃ decreases with increased temperature. The temperature can be up to about 110°C and is suitably not above 90°C in order to avoid pressure vessels.

The addition giving carbonate ions is suitably made in such a way that it becomes quickly dispersed in the pulp in order to avoid high carbonate concentrations at the wall surfaces and thereby reduce the risk for deposition of for example calcium carbonate. Thus, the addition is suitably made in a special agitator and preferably in a high intensive mixer, even though addition and mixing can be made in e.g. a pump. If carbon dioxide is added the process equipment may optionally be pressurized after the addition, in order to increase the solubility of the gas.

The method according to the invention can of course be used in any system for bleaching, e.g. ECF or TCF, of any type of pulp, e.g. softwood or hardwood sulphate pulp, where trouble- some calcium depositions might occur and where the pH values of the pulp suspension are changed for different steps according to the above. The method can of course be used for all different types of chemical paper pulp which, becuase of the original wood and/or cooking chemicals, contain calcium in such amounts that depositions are likely. When it comes to the consistency of the pulp suspension at the treatment with carbonate ions it can be the conventional one used in bleaching of pulp of medium concentration or high concentration. Suitably the consistency lies within the interval from 8 to 16%.

Claims

1. Method of reducing the content of calcium in the -liquid phase of a suspension of chemical paper pulp after subjecting it to a pH decrease to a pH under 7, characterized in that the pulp suspension is alkalized to a pH above 8 and in that carbonate ions are added to the pulp suspension in an amount of at least 0.1 kg carbonate ions by the ton pulp for formation of calcium compounds, which are adsorbed on the pulp.

2. Method according to claim 1, characterized in that carbonate ions are added by addition of a carbonate ion containing solution.

3. Method according to claim 1 or 2 , characterized in that the addition of carbonate ions is made in a high intensity mixer.

4. Method according to claim 1, characterized in that carbonate ions are added by addition of carbon dioxide.

5. Method according to any of the preceding claims, characterized in that carbonate ions are added to the pulp suspension in an amount of from 0.1 to 10 kg carbonate ions by the ton pulp.

6. Method according to any of claim 1 - 4, characterized in that carbonate ion treatment is performed at a temperature of 40┬░C or above.

7. Method according to claim 6, characterized in that the temperature lies within the interval of from 60 to 110┬░C.

8. Method according to any of the preceding claims, characterized in that the pulp that is treated is subjected to a pH reduction to under pH 6 and in that the pulp suspension subsequently is alkalized to a pH above 10.