The present invention relates to a method of fraction¬ ing peat in order to make possible an efficient and econom¬ ical utilization of one or more of the constituents of the raw peat.

Very early peat has attracted interest for industrial use. Thus, the Swedish patent 37 653 from March 15, 1912, discloses a method for extracting peat fibres and the object of that invention was to provide an economic extraction of the fibres of the peat while still in the peat moss, and to separate them from the other constituents of the moss. The idea was to obtain peat fibres for the manufacture of paper and other articles, and to reduce the costs for the extraction and separation, with a minimum of breaking of the fibres.

According to the patent a strong flow of water is directed to the peat as it is resting _^ in the moss. This brings about a cutting and desintegration of the peat layer, and a washing of the valuable fibres from roots and lumps, and the fibres are held in suspension in the utilized water. According to the patent the water is preferably directed in such a manner that there is formed a channel which leads to the place where the apparatus is disposed which takes up the water which holds the fibres suspended. This water, which holds the fibres in suspens¬ ion, forms a material container from which one is permitted to take the material in a suitable way, which material is thus held in a workable condition. It is possible to use natural water supply, if there exists such a supply in the vicinity, or use the water of the moss one or several times. At,a suitable point of this fibre and water source one may have an apparatus for extraction of the supply and for the separation of the fibres. The water which is separated from the peat during this treatment, may be collected in a suitable reservoir in order to be reused in that it is fed back to the pumps which pump the water to the jet nozzle which is used to cut out the peat from the moss.

Per se the peat extraction method according to the patent 37 653 may still be utilized but the teachings of the patent are entirely insufficient to permit achievement of a final result that would satisfy todays very high demands in technical and economical respects.

A further step is taken by the Swedish patent 45 361 from April 23, 1918, which discloses a method for treating peat by which the raw peat, having been cut up and de- watered, is moved into a basin containing a solution of an agent which has the ability to coagulate blood, such as calcium chloride or iron chloride, and is then moved further on into a rotary sorting apparatus, which sorts out the vegetable fibres which are collected in a collecting apparatus wit the aid of a flushing apparatus. Then, the flowing peat mass is caused to pass through a plurality of similar sorting apparatuses having ever finer meshes, and is subsequently passed into a suitable sorting apparatus wherein impurities are removed. Finally, the peat mass is subjected to washing and drying in air, and a heat drying, whereby a pulverous, agglomerable mass is obtained.

Thus, by the previously known method according to the patent 45 361, there is obtained one or more fibre fract¬ ions of different degrees of fineness, and a final product in the shape of a pulverous, agglomerable mass. The fibre fractions contain at least portions of the chemicals that are added during the early stages of the process, and thus there is a not neglectable consumption of these chemicals, which has an adverse effect on the economy of the process. Further, the process is comparatively energy demanding, which is similarly economically unfavourable. The degree of cleanness of the different fibre fractions and the pulver¬ ous mass are not satisfying and the water comsumption during the process is comparatively high.

The just mentioned drawbacks and different other technical and economical considerations during the passed decades have led to that the method of treating peat as disclosed in the above mentioned patent 45 361, as far as known, has never come to a practical utilization. jURt

OMP

By the extraction of fibres of peat to be used in the manufacture of paper and the like it is, of course, in most cases, desirable to be able to remove the water to a maximum possible extent. One proposal in that direction is disclosed in the Swedish patent 63 088 from October 31, 1924. This patent starts up from the knowledge that peat sets up a very high resistance to dewatering by pressing, and that this resistance to a large extent goes back to the mecous nature of peat. In the just mentioned patent it is stated generally that, if a sufficient quantity of water is mixed with the peat to sufficiently separate or disperge the solid partic- les, then the mecous gel constituent, together with the main portion of the water, will easily separate from the solid portions of the peat, and if suitable conditions are present then also the ash constituents may be separated off. In the patent it is stated that as the mecous gel has been separated off together with the easily extractable water then the remaining peat constituents may very -easily be freed from the water they still hold by a conventional pressing, and the gain from the pressing will be consider¬ ably increased.

More particularly the patent defines a method of separating water from peat or the like whereby the amount of water is first increased in order to facilitate the separation, which method comprises disperging raw or soaked ^• peat in such a great amount of water that the solid material does not exceed 2%, then in a first step, separating the solid peat constituents from the gel or mecous constituents and the surplus water by filtering, ^{• •} sedimentation, decanting or other methods, without using pressure, preferably continously, and pressing the separated fibre material in a press, which preferably works continuously, and transfering the material into a plurality of thin layers.

A technically entirely similar proposal is disclosed in the Swedish patent application 7808215-3 from Jyly 27, 1978. Also in that case it is stated to be desirable that the peat suspension should be concentration adjusted in y^ s

such a manner that a low peat fibre concentration is obtained, viz. lower than about 1 per cent by weight.

In the just mentioned patent application it is stated that the peat material suspension should be concentration adjusted in dependence of the initial degree of humidifi- cation of the raw peat, and also that the suspension should be passed to a screening member which should have a perme¬ ability which, to ensure a maximum gain from the suspens¬ ion, should be dependent on the peat fiber concentration of the suspension after the concentration adjustment.

However, in that patent application it is neither stated in which direction nor to which extent the concent¬ ration adjustment should be performed in dependence of the degree of humidification. The reason for this is believed to be that the relationship between the degree of humidifi¬ cation of the peat and the optimal concentration for the process is neither particularly strong nor constant from one peat moss to another. The limitation that the perme¬ ability of the screening member should be dependent on the peat fibre concentration of the suspension gives no guidance to those skilled in the art with regard to the design of the screening member but expresses merely a desire that the screening member should be as good as possible for use in each particular case. The present invention will be disclosed in more details below with reference to the accompanying drawing, which very diagramatically show one e obdiment of a plant for fractioning of peat according to the invention. The invention will be disclosed in connection with a description of how the fractioning process can be carried out.

Already at the outset it should be mentioned that when concentrations are discussed in the following this means, if nothing else is said, the peat fibre concentration expressed as per cent by weight of dry peat fibres in the respective concentration.

The raw peat is extracted from the moss in any conveni¬ ent manner, e.g. by digging or milling, and is placed in a intermediate tank 10. The peat may also be made pumpable alread at the m ss b m

so called hydrapulper so that it may be pumped to the intermediate tank 10. In this intermediate tank 10, which may also be called a homogenizing vessel, process water is admixed to the peat and the content of the vessel is agitated in a suitable manner so that a comparatively homogeneous suspension having a mean concentration of about 1,5-5 per cent, preferably about 2,0-3,0 per cent is obtained. The mean dwell time of the peat fibre suspension in the intermediate tank should be sufficient to permit particles and objects of higher density than that of the peat suspension itself, particularly gravel, sand and possibly some clays, to sediment to the bottom of the intermediate tank 10 to be removed from time to another. From the intermediate tank 10 the peat suspension having the said concentration of e.g. 1,5-4% is passed to a coarse screen 11 in which coarser components of the suspension, such as pieces of stumps and roots, are screened off, which pieces in a manner not shown are removed from the process as reject or a so called

0-fraction. The necessary process water is fed to the coarse screen through a pipe 12 whereby one can use pure or less pure water, depending on the economic circumstances. The accept. in the shape of a very homogenous suspen- sion having a concentration of e.g. 1,5-4% is passed through a pipe 1^ to a hole or slot screen 14 of suitable - design wherein the concentration of the suspension is increased to up around the double original value, i.e. to about 3-8% and preferably to the range of about 4,0-6,0%. Alternatively, the accept in question may be moved to a screw or roller press for dewatering to a dryness of about 50%, from which press the accept may be mixed with accept from other presses which will be disclosed below so that a slurry having a concentration of e.g. about 25% is 6b- tained.

The screening members of the hole or slot screen 14 have holes or slots dimensioned in dependence of the kind of peat suspension which should be treated and, naturally, also in dependence of the desired concentration of the

reject from the screen in question. At practical tests it has been found that a hole diameter or a slot width of from about 0,30 to about 1,0 mm and preferably of the order of about 0,5-0,7 is to be preferred at a concentration of 1*5-2,5% on the ingoing suspension and about 4-6% on the outgoing suspension. The just mentioned values may require a slight variation in adaptation to the actual conditions, such as the degree of humidification of the treated peat. It should be noted that the screen may be pressure-fed and of the open or closed type depending 'on the requirements as to capacity and/or field of use of the screen in question. It deserves mentioning already now that the releasing of the reject, i.e. the fibre rich suspension, from the screening member for transport further on, both with the hole or slot screen 14 and with other screens that will be disclosed further below, may be promoted by addition of a certain amount of spray water which then, to reduce the water volume, is preferably admitted only intermittently, under certain conditions it may be found to prefer to use compressed air or a brush for the same purpose.

From the hole or slot screen 14 the fibre rich sus¬ pension having a fibre concentration of the order of e.g. 4-6% is passed through a pipe 15 to a dehydration means, generally denoted 16. This dehydration means may consist of a known centrifuge, which is normally able to thicken the suspension to such an extent that the resulting pulp has a fibre concentration or dryness of the order of about 15-20%, which can oftentimes be regarded as sufficient if the pulp is to be used comparatively immediately and must not be stored for an extended period or transported longer distances. The dehydration means 16 may also consist of a screw press or a so called screen band press, by means of which a dryness of the order of 30% may be obtained with no difficulties. The fibre poor accept from the hole or slot screen 14 is passsed through a pipe 17 to one or more fine screens, in the present case two parallel coupled fine screens 18, 19, which may be provided with screen clothes having a mesh of 30-500 urn, preferably 80-100 urn.

It should be noted that, according to the invention, the chosen mesh of the screen clothes of the fine screens is very important for the result which may be achieved in the subsequent process step, viz. in that a significant flocculation to bring about a sedimentation or flotation can only be secured at a comparatively high degree of screening in the fine screen or screens, e.g. so that there is a dry substance concentration of about 0.7 per cent by weight, as a maximum. The relative fibre richer reject from the fine screens 18, 19 has a concentration of about 10-20%, normally about 1,5%. The fibre poor accept, which nearly has the character of a somewhat impure water, is preferably passed to a vessel 20 from where it is taken to suitable points to serve as process water. The relatively fibre richer reject from the fine screens 18, 19 is in the present case passed through a pipe 21 to a dehydration means, generally denoted 22, which may consist of a known screen band press, a centrifuge or the like. On its way to the dehydration means 22 this suspension is preferably admixed with some quantity of an additive, preferably a polymer, where the kind of polymer and the necessary amount thereof are dependent on the actual conditions, particularly the acidity and the concentration of the suspension. It has been found that with such a comparatively small addition of a suitable polymer as about 10-30 g/m of the suspension, one may achieve such proper¬ ties that the suspension easily and quickly and of course at low costs, may be thickened down to a dryness of the order of about 20-25% by means of a band press, a centri¬ fuge or the like of conventional design. The dehydrated pulp obtained from the dehydration means 22 is very dark colored, and differs to its consistency entirely from the light, fibre rich pulp which is obtained from the dehydra- tion means 16 (fraction I) . The dark colored pulp (fraction II) from the dehydration means 22 has a rather high heat value, viz. of the order 24,5 MJ/kg as compared to the heat value of the order of about 21,0 MJ/kg for the raw peat, and therefore the fraction II is extremely well suited to

be utilized as fuel in various connections. Further, it has a number of advantageous properties in connection with its use as fuel, e.g. in that the ash content is normally comparatively small. Of course, the just mentioned values may vary from one moss to another and vary with the degree of humification.

In dependence of the design of the following plant there is withdrawn from the vessel 20, besides the with¬ drawal of the necessary amount of process water, a not insignificant flow of a next to entirely fibre free suspension which through a pipe 23 is passed to a separator, generally denoted 24, which may consist of a sedimentation or flotation means. In the present case the separation is contemplated to be a sedimentation means, preferably in the shape of a known so called lamella sedimentation means. There, the suspension is admixed with suitable additives to promote the agglomeration of the agglomerable constinuents into sedimentable flocks. Such additives are, for instance, iron chloride, which may be

3 added in an amount of the order of e.g. 20-120 ml/m , suitably about 40-90 ml/m 3, and preferably about 70 ml/m3, as well as the flocculating agent which is sold under the trade mark "Magnafloc", specifically the quality "Magnafloc

351", which agent may be added in such a small amount as about 4-8 g/m 3 of the suspension, preferably about 6 g/m3.

The choice of the above mentioned additivies and the amounts thereof is dependent on i.a. pH, the temperature and the concentration, which parameters vary slightly from one moss to another and also, with the degree of humification. They are easily determined empirically by those skilled in the art.

From the sedimentation means the "clear" phase is passed througn a. pipe 25, 26 to the vessel 20 to be returned to the process as process water, or is the "clear" phase passed back to the recipient through the pipe 25, 27. I should be noted that, at practical tests, the "clear" phase from the sedimentation means has been found so free from impurities that the amount of dry substance has been less than between 10 and 100 mg/1 of the liquid in ^0MPI

analysis of the clear water has given the following values, viz. :

Suspended material 26 mg/1

BOD- 5 mg/1 COD 70 mg/1

B _tQt 0,05 mg/1

^Fe rest ' ^m *

The flocks sedimented in the sedimentation means are passed, in the lowermost portion of the sedimentation means, to a thickener, generally denoted 28, wherein a thickening of the suspension to a dry substance content of the order of 2,5-7, preferably 5-6 per cent by weight is carried out. This thickened suspension is then passed to a dehydration means of suitable' esign, e.g. in the shape of a press or a centrifuge, from which an extremely black mass having a dry substance content of 15-25 per cent by weight is obtained. In a particular case this mass has such a high heat value as of the order of about 28,9 MJ/kg and is useful per se to be utilized as fuel, and may be used for other purposes in that the properties of the mass. come close to those of coal. However, the mass can also advanta¬ geously be mixed with the above mentioned peat fraction O and II in a suitable mixer. It should be noted that a calculation of existing flows, and gains, i.e. establishing a flow balance, reveals that, apart from plant residues and stones, gravel, sand and possible clays, close to all the solid material within the moss is gained in one or another fraction whereas that which is returned to the moss is only entirely pure water. It should also be mentioned that a certain heating of the suspension, e.g. to about 60°C but not above 100°C facilitates the dehydration and permits an all around the year extraction.

OMPI