ON THE MECHANICAL DEWATERING OF PEAT HYDROMASS AND THE PRODUCTION OF MARKETABLE PEAT FROM IT WITHOUT SPREADING ON DRYING FIELDS

I. V. GEBLER, and V. N. PONOMAREV

(Presented by the joint seminar of the Faculty of Chemical Technology)

The technological process of producing peat fuel generally consists of the following sequential operations:
1) extraction of peat from the natural deposit; 2) transport of peat to drying fields; 3) drying of peat in the fields under natural conditions; 4) harvesting of dry peat into storage units.

The first operation in hydraulic peat extraction is carried out by breaking up the peat in the deposit with a jet of water from a hydraulic monitor, striking under a pressure of 14–15 atmospheres. In this process the peat is converted into the so-called hydromass, containing 4–6% dry matter and possessing the mobility of a liquid. This mass is taken up by a peat pump, by which it is delivered through a pipeline to drying fields. The peat pump is equipped with a grinder for mechanical processing of the mass.

This method of peat extraction has major advantages, namely:

1. Complete mechanization of the process.
2. The possibility of carrying out work at any degree of stumpiness.
3. Complete extraction of peat from the deposit without omissions (which occur with the excavator method).
4. No need to prepare the deposit and drain it to the extent required in the excavator and milling methods of peat extraction.
5. Transport of the extracted peat in the form of a flowing hydromass is carried out easily and simply through pipelines by means of pumps, and is possible over large distances in comparison with the transport of excavator peat, for which the travel distance is limited to 200–300 m. This circumstance simplifies and intensifies the system of operating peat bogs, especially large ones.

However, the hydraulic method of peat extraction has its own significant disadvantages, all of which are concentrated in the third operation—the drying of peat and the accompanying work in the fields.

These disadvantages are as follows:

1) extension of the drying process by 10–15 days in comparison with excavator peat;

2) limited choice of spreading fields, which must have good filtering capacity and a definite slope not exceeding 0.002;

3) in equipping hydropeat enterprises, far more metal is expended than with other methods, namely: per 1000 tons of annual output (without transport), with the milling method—2 tons, with the excavator method—4 tons, and with hydropeat—8 tons of metal;

4) the total energy consumption in the hydropeat method, compared with the excavator method, is considered to be 2.2 times greater.

All these disadvantages together lead to the fact that hydropeat is the most expensive; it is 1.3 times more expensive than excavator peat and 2 times more expensive than milled peat.

The advantages that exist when extracting peat from the deposit in the form of a hydromass lead to the question of its mechanical dewatering during extraction, so as to bring the mass quickly to the lowest possible moisture content or, in some cases, to the moisture content at which the peat is required in certain instances of its use.

Extensive work on dewatering hydromass was carried out by Gidrotorf [1]. The main conclusions from these works are as follows. In the process of water release by the peat hydromass, three stages occur.

First stage. From a moisture content of 96.5–97% to 95–95.5%. Water can be removed by settling or by filtration in open filters without vacuum. Filtration proceeds rapidly. The process is accelerated when the hydromass is coagulated with colloidal iron oxide. The application of a vacuum on the order of 0.1–0.15 bar also greatly accelerates the filtration process, but is impractical because of the bulkiness and cost of the apparatus for this first dewatering, in which 30 to 50% of all the water contained in the hydromass is lost. For this stage of dewatering, Gidrotorf proposed elevators with mesh buckets and filtering drums. The former, for coagulated hydromass, removed 9–10% of all the hydromass water at a dry-matter content in it of 4.15% and a capacity of 16.4 tons/hour. A decrease in capacity and dilution of the mass increase the amount of water removed.

A filtering rotating drum with a mesh surface serves for further dewatering, bringing the dry-matter content to 5.5%.

Second stage. Water is removed under a pressure of 0.5–2 bar or under a vacuum of 0.4–0.7 bar. Dewatering is brought to a moisture content of 88–81% for coagulated hydromass. The high figure (81%) is achieved at a very low productivity of the apparatus and a large energy consumption, which reaches 30–40 kW per 1 ton of dry peat substance when peat with a moisture content of 85–86% is discharged, and with a productivity of up to 60 kg/hour of absolutely dry peat per 1 m² of filtering surface.

The process of filtering hydromass can be carried out on continuous vacuum filters of various designs: drum, disk, and plate filters. The use of filters has major shortcomings.

Third stage. At this stage, peat after vacuum filters must be dewatered from a moisture content of 83–86% to 63%.

This process is carried out by pressing the peat mass in hydraulic presses at a pressure of 30–50 bar, with a specific loading on the order of 6 g per 1 cm² of filtering surface for 3–5 minutes.

It is indicated that the upper limit of dewatering is achieved only when about 10% dry peat powder (the “Madruk” method) with a moisture content of 15–18% is added to the peat mass. In this case additional processes are necessary: thermal drying of the peat to obtain 10%

(of the weight of the peat after the filter) of powder, its grinding until it passes between sieves with 600 and 3000 openings/cm² and, finally, mixing the powder with the peat (dusting).

Later, the Moscow Peat Institute [2] developed the design of a continuous high-pressure batch hydraulic press (up to 12 bar) in combination with sawdust filters for dewatering raw peat or thickened hydromass with a moisture content of 86–80% to a moisture content of 55%; the pressing time is 10–12 minutes, the specific load is 0.25–0.5 g/cm², and the daily output is 325–650 t of peat with 55% moisture at an energy consumption of 27 megajoules per 1 t of such peat.

Thus, mechanical dewatering of the hydromass itself, within a short time on the order of 1–2 hours, to a final moisture content of 55–63% is a complex multistage process involving the use of apparatuses differing in principle of operation and design, sometimes complex and of low productivity.

Methods of dewatering that include thermal treatment are completely unsuitable for hydromass, given a negative heat balance.

The degree of mechanical dewatering of peat, generally speaking, may be required to vary: in some cases to the maximum possible limit for subsequent thermal drying (55–60% moisture), sometimes to the practical possibility of thermal treatment (70–75%) for further deep dewatering by pressing to a moisture content of, for example, 37%. If lump peat is needed, then for this purpose forming in a machine will require a moisture content on the order of 86–80%, depending on the properties of the peat. In this case the advantages of extracting peat from the deposit in the form of hydromass and the benefits of transporting it can be combined with the production of peat bricks.

Working in this direction, the author came upon a method of mechanical dewatering of hydromass in which the processes of filtration and pressing take place in one continuously operating apparatus, whose principle is borrowed from the technology of paper production [3].

Paper pulp, from which paper is made, is a highly watered mixture of fibers of wood pulp, cellulose, and sometimes cotton-paper rags with certain fillers. The quantity of dry substances in such a mass is 0.5–0.6%, i.e., it is diluted 8–10 times more than peat hydromass. Dewatering of paper pulp and the production of the paper sheet are carried out on a paper-making machine. The first part of the machine along the path of the mass is a metal wire, moving on wire-guiding rollers at a speed of 0.8–3 m/sec and higher. For better action of the wire with respect to dewatering, so-called suction boxes are arranged beneath it. They are boxes located across the wire, filled with water; when the water is removed by a pump, a vacuum of 0.25–0.17 bar is created.

On the wire the mass is dewatered to a content of 2.5% dry substance. Then the wire with this mass passes through the so-called couch press, which consists of two rolls arranged across the wire; of these, the lower roll sets the wire in motion, rotates in fixed bearings, and serves as a support for the upper roll, which, rotating in bearings attached to movable arms of levers, lies directly on the wire and by its weight creates a pressure of about 30–50 newtons per 1 cm (linear) of wire width, depending on its width. The upper roll is covered with a woolen sleeve. After

the couch-press paper web contains 20–25% dry solids, leaves the wire, and is transferred to wet felt, on which it is then successively subjected to the action of three more ordinary presses, analogous in design to the couch-press but with higher pressures than the latter: press I—25%, II—50%, and III—75%.

After press III the paper web contains 35–40% dry solids and, leaving the wet part of the machine, proceeds further to drying.

In terms of mechanization and coordination in the operation of many separate units when they are automated, modern papermaking machines are very advanced apparatuses.

The wet part of these machines may serve as the basis for mechanical dewatering of peat hydromass by filtration and subsequent pressing, with simplifications and modifications in accordance with the properties of peat.

A machine for peat must likewise be a wire that moves over rollers, bending around the front roll and the lower press roll, analogous to the couch roll of a paper machine.

The peat layer, having given up part of its water on the wire, approaches the press in a condition that permits pressing under a certain pressure and, being squeezed between the rolls to a moisture content of 86–84%, is removed by a scraper from the upper roll. If intended for molding, such peat may go directly to the molding machine.

If further dewatering is required, then after the first pair of rolls the peat passes onto an extended wire and is squeezed between a second pair of rolls, which can now be done at high pressure, and this yields a product with a moisture content of about 75%. After the last press, on the return run the wire is completely cleaned by spraying with water.

Experiments were carried out on a laboratory model of a machine having the following arrangement (Fig. 1). A wire with openings of 0.25 mm is stretched

Fig. 1. Laboratory model of a machine for mechanical dewatering of peat.
1—wire, 2—front roll, 3—lower press roll,
4—upper press roll, 5—suction box, 6—water drain line,
7—line to the pump

between two rolls; the width of the wire between the enclosing rubber belts is 60 mm.

The rear roll in the direction of travel is the driving roll; on it rests the upper roll, weighing about 1 kg, rotating in the sockets of unequal-arm levers fastened on hinges in the frame. By suspending a load from the long arms

the levers creates the necessary pressure, which is concentrated along the generatrices of the rolls.

The length of the wire mesh between the axes of the front and upper rolls of the press is 20 cm; in the experiments the mesh speed ranged from 0.5 to 1 cm per second. A slight vacuum (0.1 bar) is created by means of a box beneath the mesh along which it slides.

After the first pressing, the peat layer was removed from the roll, placed on a clean mesh, and a second pressing was carried out.

The work was performed with peat from the Tagan deposit (near Tomsk); the peat was hypnum-sedge peat, with a degree of decomposition of 30–35%. The peat was cut from the deposit in the form of pieces with a moisture content of 88–89%.

The hydromass was always prepared with a content of 4% dry matter and was not rubbed, since, as is known, this greatly worsens the filtration capacity of peat. We consider that, in laboratory work, passing the mass before filtration through a grinder, as was done in the experiments of Gidrotorf, is extremely harmful, and we always adhered to the following procedure. The raw peat was torn by hand into small lumps; the required amount of water was added to it, and the resulting hydromass was rolled with a rubber roller in a flat cuvette, on the bottom of which a rubber plate had been placed, until the lumps disappeared. This kind of elastic kneading of the peat showed great advantages in preparing the hydromass and accelerated its filtration, as was established in the experiments.

In working with the laboratory machine, the following observations were made for hydromass containing 4% dry matter.

Removal of water on the mesh proceeds rapidly: in 10–30 seconds peat with a moisture content of 90–91% was obtained, at which it was well dewatered by the rolls. At a pressure of 15–16 newtons/cm, peat with a moisture content of 85–86% was obtained, which was removed from the upper roll in the form of a dense layer. Increasing the pressure to 27 newtons/cm gave a product with a moisture content of 81–82%. With this moisture content (within the indicated limits), the peat can proceed to molding in screw presses, like excavator peat.

For further dewatering, the peat layer on the mesh was subjected to pressing on the second pair of rolls at a higher load, from 110 to 180 newtons/cm, which gave peat with a moisture content of about 77–71%.

The experiments were carried out without coagulation of the hydromass and without dusting the peat with dry peat powder.

In production, the following scheme is envisaged (Fig. 2). Hydromass from the quarry is fed by a peat pump, receiving moderate mechanical treatment, into a collecting accumulator, from which it enters a reservoir with an agitator and then flows by gravity onto a mesh with openings, for example, of 0.25 mm. Under the mesh there are suction boxes located between the rollers supporting the mesh. The hydromass, being filtered on the mesh and dewatered to 90–92% moisture, approaches the rolls of the press in a state suitable for pressing, and is dewatered to the moisture content required, for example, for molding in a screw press: 84–86%. The peat layer is removed from the upper roll by a scraper. On the return run the mesh is cleaned by spraying with water.

Suppose that the mesh width is 2 m. The thickness of the hydromass layer poured onto the mesh is 0.05 m. The mesh speed is 1 m/sec. If dewatering of the mass on the mesh by filtration is carried out to 91% moisture, then the thickness of the peat layer approaching the press rolls will be about 0.022 m; in this layer the peat will enter pressing between the rolls, admis-

Fig. 2. Technological scheme of an industrial installation for mechanical dewatering of peat.
1 — intermediate reservoir for hydromass; 2 — screen; 3 — guide roller; 4 — separating rolls; 5 — suction boxes; 6 — couch press; 7 — [[unclear: “dekel”/“lekel”]]; 8 — forming machine.

to 85% moisture and emerge from under the rolls in the form of a ribbon about 0.0133 m thick. The productivity for product dewatered to 85% moisture will be 95.7 t/hour, which corresponds to 14 t/hour of absolutely dry peat (not counting losses). The process proceeds continuously.

The filtration length of the screen is determined on the basis of experiments, from the condition that, when the hydromass is filtered through 1 m² of screen, about 0.055 t of filtrate passes in 10 seconds. With a screen width of 2 m and a speed of its movement of 1 m/sec, its length is 5 m.

In the same installation, the process of obtaining peat in the form of flakes can be carried out. Such peat is similar to milled peat in the shortness of its drying time.

For this, the screen must be made of great length, and along its continuation after the first press one or two more presses must be installed with the same weight of upper rolls, so that after the last press the peat has a moisture content of the order of 75–71%. In this state it will be removed by a scraper from the screen as it bends around the lower roll of the last press, in the form of a dense layer breaking up into plates of irregular shape. They are directed into the hopper of machines (similar to those used for harvesting milled peat) and transported to the drying site. Under favorable weather conditions, peat in this form will probably dry in 2–3 days.

The energy consumption for dewatering on the machine described, in the absence of experimental production coefficients, can be approximately determined from data for paper-making machines using empirical formulas [4]. According to these formulas, the energy consumption for dewatering was determined by us as 13 megajoules per 1 tonne of absolutely dry peat.

In the book Hydrotorf [1], for the energy consumption for the product after filtration with a moisture content of 83–86%, i.e. analogous to that obtained in our experimental machine with one pair of rolls, the following figures are also given, calculated per 1 t of peat with 18% moisture:

Conclusions

1. The hydraulic method of peat extraction, which at present has unfavorable technical-and-economic indicators, can be improved at the drying stage.

2. A method has been developed for mechanical dewatering of hydromass, distinguished by simplicity of technology and structural design and, evidently, by economy.

LITERATURE

1. Hydrotorf. Artificial dewatering of peat by the Hydrotorf method, part II. Hydrotorf Publishing House, 1927.

2. S. A. Tsyprov. Mechanical method of peat dewatering. Peat Industry, No. 2, 1947.

3. Production of semifinished products and paper. Paper-making machines, vol. II. Bumburo Publishing House, Moscow, 1928.

4. Paper-Maker’s Handbook. Scientific-Technical Sector of the Paper Industry, Moscow, 1929.