EP1786974B1

EP1786974B1 - Device and method for production of cellulose-based products

Info

Publication number: EP1786974B1
Application number: EP05783356A
Authority: EP
Inventors: Torbjörn Hansson; Per Tomas JÄRNMARK; Olofsson Greger
Original assignee: Jarnmark Tomas
Current assignee: Jarnmark Tomas
Priority date: 2004-09-10
Filing date: 2005-09-12
Publication date: 2010-11-03
Anticipated expiration: 2025-09-12
Also published as: WO2006041369A1; ES2355799T3; DE602005024576D1; EP1786974A1; PL1786974T3; US20090277598A1; SE0402178D0; ATE486999T1

Abstract

The present invention relates to a device and a method for increasing the yield, as well as for enabling an increase in productivity and an increased amount of production, and at the same time to improve purification of process water and waste water, respectively by decreasing the waste water flow and the amount of pollutants in the waste water.

Description

TECHNICAL FIELD

The present invention relates to a device and a method for increasing the yield (getting more final product from a given incoming amount of raw materials and additives), as well as for enabling an increase in productivity (amount per time unit) and an increased amount of production (amount of production per given amount of pollutant to an end recipient), and at the same time to improve purification of process water and waste water, respectively, by decreasing the waste water flow and the amount of pollutants in the waste water. The method comprises increased recycling (recovery, closing) of process water, in combination with utilization of substances to be included as materials in an end product formed, instead of constituting pollutants in the waste. At the same time, equivalent or improved dewatering properties are achieved in the production process, thereby enabling increased productivity and/or amount of end product formed, in relation to the maximum amount of effluent to the recipient allowed for the activity.
These effects are achieved by introducing, in or between any one or several of the water processing steps of the entire production, a new step based on an effect that can be achieved by certain strong oxidizing agents, preferably ozone. The method means hereby that the oxidizing agent functions both as a type of ionizer (polarizer) of, as well as a type of catalyst for polarization between, particles, substances and additives suspended in the water. The effect is influenced by factors such as the concentration of oxidizing agent, the degree of installation, position and order of water flows in the process, mixing methods, suspension characteristics of pollutants, choice and properties of chemical additives (retention agents, etc.), dosage of additives and dosing positions, as well as other parameters. Hence, the said effect goes beyond prevailing and common effects that can be achieved by strong oxidizing agents and that are used, to a limited degree, in this type of industry in order among other things to kill microbiological cultures, to decompose pollutants comprising COD and BOD, to bleach substances in the end product, etc., as is known from FI 110683B , e.g.
The method results in an economical gain, by decreasing the discharge of substances (pollutants) in the waste water, instead to be recycled to the process as raw materials, and by enabling an increased productivity and/or production capacity by improved dewatering properties for the web of material formed during production.

BACKGROUND

The following background and description focuse on process water in pulp and paper industries, but the invention is not limited to the fields of application used to exemplify the technique and its use. The technique is applicable on other types of process industries having substances present in the process water.
Large amounts of water are used in the production processes of pulp and paper industries. It is an objective for this sector of the industry among other things to reduce the amount of pollutants discharged, as well as the amount of water discharged (waste water), also meaning that the amount of externally added fresh water is reduced. There is a strive to achieve this by increasing the amount substances used in the production, which substances would otherwise be released in the form of pollutants in order thereafter to be made subject for further purification, decomposition and final depositing in the environment, as is for example shown in EP 928842 , wherein a flotation process is used to reduce the final amount of deposited material in connection with recycling of paper.
This objective arises by a) gradually increased requirements, established by the authorities and relating to amounts of pollutant effluents allowed from the process industries, as well as b) the need for the industry constantly to try to increase productivity and profitability by increasing the utilisation of added raw materials, decreasing the amount of cost-increasing additives, and by trying to increase production.
The demands on decreasing of effluents are constantly raised, both as a consequence of common and constant technical improvements, but also in order to encourage new technical improvements with the aim of environmental control and a decreased environmental impact. Therefore, it is an important and central issue to the line of business to develop new methods for decrease of effluents and for improvement of profitability, whenever compatible.
As such, decreased effluents give the possibility to increase production within allowed/- authorized effluent limits. Nowadays, various types of environmental certificates are awarded if you are below stipulated limits, which may result in marketing benefits.
The industry strives to accomplish so called closed systems in the production process, concerning internal process water, which means that efforts are made to re-use (recycle) most possible of the process water, thereby to minimize the amount of waste water and of course consequently also the amount of added fresh water. In this context, waste water means material-containing (polluted) process water that is led to an external purification plant outside the process plant, and thereafter, after processing, to the recipient.
The quality of recycled process water tends always to be gradually impaired at increased "closing" or re-use (degree of recycling) in the plant/mill. In turn, this leads to a gradual quality impairing effect, both on the actual production process and on the product produced in the plant/mill. This is a central problem.
External waste water purification takes place by a combination of chemical, physical and biological purification methods. It is of utmost importance to minimize the amount of pollutants in the waste water as well as the amount of waste water, in order to minimize investment and operational costs for a purification plant.
Generally, it can be said that water treatment and water purification within pulp and paper industry comprises three separate steps or parts. The first part is cleaning and treatment of incoming raw water for the plant. Generally, such water is taken from a lake or a large stream.
The second part comprises purification of the internal process water, and the third part comprises purification of polluted water that is not intended to be re-used (closed) in the process and that therefore is led away therefrom as final waste water.
Generally, this means that the second step takes place within the plant and that the third step takes place outside the plant, in a dedicated waste water purification plant. The measures for the second step purification (within the plant) could partly be of the same basic technical nature as the measures undertaken in the third step, but could also be of totally different nature.
The pollutants in the form of suspended substances and acid-consuming substances, leaving the production process by the waste water, consist to a major part of raw material substances such as cellulose material and additives. These substances would have given an increased production yield if they could have been better kept and utilised as a part of the end product instead of leaving the plant by the waste water.
Internal measures to decrease effluents in the form of waste water for external purification include modifications of the production processes, in order to generate less effluents, as well as various methods for recycling internal process water (so called white water) used in the plant, in order to recycle raw materials in such water to the produced end product.
One example of internal purification of water for recycling, is purification of internal process water around a paper machine. Paper production requires a lot of water, since the stock (water plus fibre plus additives, etc.) must be much diluted before it is lead onto the paper machine wire. Thereafter, the water is drained, partly on the wire (the strainer screen on which the paper is formed), and partly in the press, section of the paper machine. This drainage water is loaded with cellulose material and other suspended materials, as well as dissolved organic and inorganic substances.
Fresh water is constantly supplied to the process water system (which also means that roughly the same amount of water leaves the system in the form of waste water). The major part of the process water is recycled to the paper machine. This closing decreases water consumption and loss of raw materials.
It is mainly the possibility of effective separation of substances in the process water that limits the degree of recycling of the process water. At poor separation of substances from the process water, or at an increased degree of recycling, accumulation of materials take place in the process water, and this may lead to problems in the form of formation of slime, clogging, as well as impaired production and product qualities.
Usually, flocculants are charged to increase retention of materials in the formed product. It is important in this case that the flocculation results in flocks of material with beneficial dewatering properties, since the dewatering is of decisive importance to the productivity of a paper machine.
Today, purification of the internal process water to be recycled usually takes place in screens, filters, settling basins and flotation plants.
Purification techniques of today, of waste water from the plant, in external purification plants, are based on a combination of mechanical, chemical and biological purification steps. By biological purification (biopurification) is meant the use of cultures of micro-organisms in order to decompose pollutants and acid-consuming materials, other bacterial cultures, etc.
It is also known in a few pulp and paper mills to use ozone in the last (external) purification step, after extensive biological treatment. In these cases, ozone is above all used in large amounts to oxidize high-molecular materials, being very hard to decompose in a biological purification plant, into substances of lower molecular size that can thereafter be decomposed in a subsequent biological purification step.
It is important to avoid large variations in the load of the external purification plant, since this has to be dimensioned for a maximum load, and since variations will, as such, result in disturbances in the external purification process.
It should be mentioned that by techniques of today, purification steps outside the production in the actual plant/mill, constitute an economical load on the total result for the pulp and/or paper production.
In some applications, ozone is also used to kill bacteria, see FI 110683B e.g. In this application, a method is described of using ozone to kill micro-organisms in the internal process water in paper mill.
There is a constant need for improved purification techniques in the mills, and the need increases concurrently with a gradually increased environmental consciousness in terms of more stringent rules for effluents allowed from the mills. Of special interest are techniques also having the possibility to improve purification of process water within the mill (within the production process), and that allow for increased recycling of process water at maintained quality for the production process and for formed products, and thereby decreased requirement of fresh water supply and decreased waste water discharge.
If the technique moreover can be formed such that material separated from the process water in the purification steps can be recovered as a marketable product, i.e. be completely or partly recycled in the production process, instead of, as now, constituting a load on the external waste water purification, then an economically profitable incentive has been introduced for a gradual improvement of the purification technique. A cost has been converted into "profitable" recovery, as well as increased productivity, i.e. a purification that pays its own cost or even more.
Accordingly, there is a need for new technique that improves the production economy (profitability) of the mills, thereby ensuring fulfilment of stipulated outlet limits, and also ensuring quality demands for the production and its product, by fulfilling at least one/some, preferably all of the following criteria:

a decreasing the amount of waste water
b decreasing the need for incoming raw water (fresh water)
c decreasing the need of or the cost for depositing
d recovering of useful/valuable substances from recycling and waste water, respectively
e decreasing loss of raw materials (otherwise leaving as pollutants in the effluents)
f allowing for increased productivity.

BRIEF DESCRIPTION

The present application relates to the accomplishment of chemically affecting the substances in the process water, thereby becoming more reactive in order to accumulate to form larger aggregates, so called flocculation, and also so that the substances will more easily react with chemicals added to improve flocculation, the now said being achieved by mixing in an oxidizing agent such as ozone gas in the process water. In the present invention, an oxidizing agent such as ozone is used for a different purpose than what has been done previously in the process line. The oxidizing agent may be ozone or some other oxidizing agent having an potential of oxidation of preferably above 1.5 V.
By adding an oxidizing agent such as ozone integrated with the process, several advantageous effects are achieved.
As mentioned above, ozone has been used previously in external purification in order to improve separation of COD. Ozone has also been used as a bactericide in the internal process water, and as a bleaching agent. In the present application, ozone is used to achieve a flocculation, or to render it more effective, of substances in the flow of fibre pulp as well as in the process water. By adding ozone as close as possible to the forming step (the effect will be better the more close to the forming step that the ozone is added), a larger amount of the substances will be caught in the network of cellulose, i.e. in the product, and thereby materials are recovered from the process water, which materials would otherwise be lost as pollutants, also resulting in the white water becoming cleaner, such that a larger part of the white water can be recycled in the short and long circulations. Besides this, combinatory advantages are attained in that also the bactericidal properties of the oxidizing agent can be used. Moreover, by using the bleaching properties of the oxidizing agent, a possibility is achieved to change the colour of excessive sludge, thus enabling this to be mixed into the raw material pulp, without affecting the colour of the end product.
The result thereof is a decrease in the amount of substances that leave the production by the waste water, which means an increased yield in relation to used raw material, as well as a decreased load on the external purification plant, resulting in an economical contribution to the pulp and/or paper mill, instead of becoming a costly problem of depositing.
The process water quality is also generally improved by addition of oxidizing agents, preferably ozone, thus enabling decreased water consumption for the production process for paper and pulp (increasing the degree of recycling, i.e. increasing the degree of closing). Bacteria and bacteria growth is reduced, otherwise leading to odour, formation of slime and disturbances in substance flocculation.
The improved flocculation properties that can be achieved by addition of oxidizing agents, will also result in an improved function (increased degree of separation) for the mechanical equipment for treatment of process water, such as screens, filters and similar. At the same time, the function is also improved for settling and flotation equipment, intended for separation of materials from the process water. In particular, it is fragments of cellulose fibres and other particulate substances that in this way are more efficiently separated from the process water.
The flocculation of substances and the process of forming fibre material into a fibre network, in all steps of the production processes within pulp and paper industry, is of central importance to the production result. The addition of an oxidizing agent will give a beneficial structure of the fibre network during forming of the network, thus increasing retention of substances in the network, and at the same time, separation of water from the fibre network is facilitated due to the changed network structure. Thereby, utilisation of the material is improved (increased retention), as well as so called runnability (less frequent disturbances during production) for the various process steps in pulp and paper production.
By the measures above, i.e. the oxidation treatment of the process water, there will be a decrease in the amount of pollutants constituting a load on the external waste water purification. Still however, some excessive sludge will be formed in the external purification step. Since the treatment with an oxidizing agent is bactericidal, besides being a catalyst for aggregation of the substances, a recycling to the (paper) production process can take place by oxidation treatment of this sludge material. The effect will be bacteria-reduction in the sludge and aggregation of the substances, to be caught by the production process, in which it is screened out with the product.
The treatment with the oxidizing agent is also colour-reducing, resulting in that the most often darker colour of the sludge can be reduced to a suitable nuance, as desired.
A higher dosage of the oxidizing agent will be more colour-reducing than a lower dosage. This affects the possibility to recycle the sludge as a part of the products, without affecting the colour of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1: shows a schematic drawing of material and water flows in pulp and paper production, in which an ionization and oxidation step 12 has been introduced in the process,
Fig. 2: shows an embodiment example of the mixing-in of ozone gas according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail, with reference to the drawings.
Fig. 1 shows a schematic drawing of material and water flows in pulp and paper production, in which an ionization and oxidation equipment 12 has been introduced in the process, preferably just before the forming equipment 14. Raw materials 1 and water 2 are supplied to the production process 3. The product 5 is produced and water vapour 4 and polluted output process water 6 leaved the production process 3. The output process water 6 is purified in an external purification plant 7 for waste water, from which one part leaves as sludge 8 for deposit and/or incineration, and the purified water 9 is let out to the recipient (i.e. the environments). Looking closer into the production process 3, it can be seen that raw material 1 and water 2 are supplied to a pulp and stock preparer 10. Also recycled process water 15, 20 is supplied to the pulp and stock preparer, from the storage tank 19 for white/process water and the forming equipment 14 (preferably a paper machine), respectively. The pulp/water mixture 11 resulting form the pulp and stock preparer 10, is thereafter exposed to an ionization and oxidation step 12 (see Fig. 2). The ionized and oxidized output water 13 from the ionization and oxidation equipment 12 is thereafter introduced in the forming equipment 14. Preferably, the water flow 11, 13 is also treated with retention agents. In the production line, the distance between the pulp and stock preparer 10 and the forming equipment 14 can be relatively large. It is preferred that the ionization and oxidation equipment 12 is situated more or less in direct connection with the forming equipment 14. The forming equipment 14 produces the paper product 5 by distributing the treated, fibre-carrying water 13 (the pulp suspension) onto a forming wire. A part 15 of the white water 15, 16 from the forming equipment 14 is recycled directly to the pulp and stock preparer 10, in the so called short circulation, another part 16 goes to internal purification equipment 17. Furthermore, water also leaves as water vapour 4 that is let out. The internal purification equipment 17 may for example contain various screening steps and/or flotation steps, in which a part 18 of the water can be re-used and therefore be sent to a storage tank 19 in order to be recycled to the pulp and stock preparer 10. The water 6 that contains a higher concentration of reject material than water 16 and 18, and that is not led to the storage tank for process water 19, is sent to external purification equipment 7 (such as flotation basins) that separates bio-sludge 8 and water 9 that is considered to be adequately purified and that is therefore returned to the environments. Under certain circumstances, parts of the bio-sludge 8 can be returned to the production process 3 at the ionization and oxidation equipment 12.
One embodiment example of the mixing-in of ozone is shown in Fig. 2, i.e. the ionization and oxidation equipment 12 that is inserted between the pulp and stock preparer 10 and the forming equipment 14 (see Fig. 1). A pulp/water mixture 11, typically having a pulp concentration of between 0.5 and 5 %, is pumped into a container 27 (the mixture is normally diluted with internal process water that in turn contains pulp concentrations of normally below 0.5 %). Ozone gas 24 is added to the bottom of the container 27, by being injected into the container 27 in the form of small bubbles 25, in order thereby to increase contact surfaces against the pulp/water mixture 11. The ozone gas 24 can be generated by using an ozone generator. Preferably, a stirrer 23 is positioned inside the container 27, in order to stir the pulp/water mixture 11 and further to increase the possibilities for the ozone gas bubbles 25 to react with the substances in the pulp/water mixture 11. Appropriately, other stirring means for the pulp/water mixture 11, can be used. The top of the container 27 is provided with outlet pipes 21, 22 for remaining ozone gas 24 that has passed the pulp/water mixture 11 without having reacted. The remaining ozone gas 24 can either be led via pipe 21 to an ozone-destroyer, or it can be returned to the ozone generator via pipe 22, for re-use in order thereafter to be recycled to the container 27.
In order to achieve a result that is as good as possible, it is preferred for the ozone gas 24 to have a long contact time with the process water. Therefore, the ozone gas 24 is preferably added in the above described reactor vessel 27. However, it can also be added in a longer pipe in which the ozone gas 24 is mixed with the pulp mixture 11. Preferably, the process water 11 should be stirred during the reaction time. The dwell time for the liquid 11 in vessel 27 is a function of the volume of the vessel 27 and the liquid volume flow through vessel 27. If the concentration of ozone is high in the ozone gas 24, the contact time may be shorter in order to achieve the same effect of the ozone gas (the so called Ct factor, where gas concentration and reaction time is a product factor that weighs the importance of changes in gas concentration and reaction time, respectively). Stirring can take place either by a stirrer 23 in a container 27, or by a so called static mixer in a pipe through which process water 11 and ozone gas mixture 24 are pumped. A combination of a container 27 with a stirrer 23 and a pipe with a static mixer, is also possible. The ozone gas 24 can be added e.g. by being bubbled 25 into the process water 11, at as small gas bubbles as possible, in order to achieve a contact surface with the process water 11 that is as large as possible. Mixing-in can take place e.g. by bubbling equipment, e.g. acting from the bottom of a container 27 in which the process water 11 is contained. Mixing-in can also take place via a mixing-in pump such as a type of turbine pump, or by adding ozone gas 24 into a pipe by aid of a so called ejector or by a dosing lance. The mixing of the process water 11 and the added ozone gas 24 should be as good as possible. If it is desired to recycle parts of the bio-sludge 8 that is separated in the external purification 7 (see Fig. 1), parts of the sludge 8 can be recycled to the container 27 in which the ozone gas 24 is added to the process water 11.
In the brief description above, the principles for the action of the ozone gas are thoroughly discussed.
In the description above, the method has been described when using ozone gas. Naturally, other oxidizing agents can be used at greater or less extent, and in combination. It is also conceivable that the part of the white water 15 from the forming equipment 14, that is recycled directly to the pulp and stock preparer 10, is exposed to an ionization and oxidation step. Furthermore, water 20 from storage tank 19 can be exposed to an ionization and oxidation step in direct connection with the pulp and stock preparer 10.

Claims

A method for rendering the flocculation more effective in the production of cellulose-based products, comprising the steps:
a) a step for pulp and stock preparation (10), into which raw material (1) and fresh water (2), as well as recycled white water (15, 20) from step c) down below, are supplied in order there after to be mixed together,

b) a subsequent ionization and oxidation step (12), in which the pulp/water mixture (11) of step a) is subjected to ozone gas (24), where the ozone gas functions both as an oxidant and as an ionizer chemically affecting the substances in the pulp/water mixture (11) to become more reactive which thereby forms larger aggregates rendering the flocculation more effective in the pulp/water mixture (11),

c) a forming step (14) in which the end product (5) is produced from the mixture exposed to step b).
A method according to claim 1, characterised in that a retention-improving step is inserted between step a) and step b), by which retention agents are supplied to the pulp/water mixture (11) from step a).
A method according to any claims above, characterised in that a retention-improving step is inserted between step b) and step c), by which retention agents are supplied to the pulp/water mixture (13) from step b).
A method according to any claims above, characterised in that the incoming recycled white water (15, 20) of step a) is at least partly subjected to ozone gas (24).
A device rendering the flocculation more effective in the production of cellulose-based products, at least comprising a pulp and stock preparer (10) as well as forming equipment (14), a pulp/water mixture line (11, 13) between the pulp and stock preparer (10) and the forming equipment (14) arranged to supply the forming equipment (14) with a pulp/water mixture (11) from the pulp and stock preparer (10), further at least one recycling line (15; 16, 18, 29) arranged to recycle white water (15, 16) from the forming equipment (14) to the pulp and stock preparer (10) characterised in that a first ionization and oxidation equipment (12) is arranged to supply ozone gas (24) to the pulp/water mixture line (11, 13), where the ozone gas (24) functions both as an oxidant and as an ionizer chemically affecting the substances in the pulp/water mixture (11) to become more reactive which thereby forms larger aggregates rendering the flocculation more effective in the pulp/water mixture (11).
A device according to claim 5, characterised in a second ionization and oxidation equipment (12) is arranged to supply ozone gas (24) to the at least one recycling line (15; 16, 18, 29), preferably in close connection to the pulp and stock preparer (10).
A device according to any of claims 5 to 6, characterised in a that the ionization and oxidation equipment (12) comprises a reactor container (27) having an inlet arranged to receive a pulp/water mixture (11), an ozone bubbling inlet arrange to bubble (25) ozone gas (24) in the lower part of the reactor container (27), a stirrer (23) arranged to mix the pulp/water mixture (11) and the ozone gas (24), a first outlet for the ionization and oxidation treated pulp/water mixture (11) and second outlets (21, 22) at the upper part of the reactor container (27) arranged to receive any remaining ozone gas (24).
A device according to any of claims 5 to 7, characterised in that the first ionization and oxidation equipment (12) is inserted to the pulp/water mixture line (11, 13) in close connection to the forming equipment (14).