RU2797281C1 - Waste storage method - Google Patents

Abstract

FIELD: hydraulic engineering.

SUBSTANCE: invention can be used to create storage tanks for domestic and industrial waste. The method includes preparing the base by backfilling the draining soil, laying a waterproof screen, backfilling the protective layer of soil, supplying it to the accumulator and subsequent pumping out liquid to check the tightness of the screen, while a fine clay suspension is used as a liquid, into the draining soil and the soil of the protective layer during backfilling flocculants are introduced, and Portland cement is introduced into the suspension of fine clay, after which it is fed into the storage tank, in an amount of 0.4-1.2% by weight of the suspension. As a suspension of finely dispersed clay, saponite-containing kimberlite ore enrichment waste can be used, and chalk is used as a flocculant introduced into soils.

EFFECT: invention makes it possible to increase the operational reliability of the waste storage tank by ensuring the integrity of the watertight screen, as well as to reduce the time for putting the tank into operation due to the combination of operations of checking the tightness of the screen and eliminating existing defects.

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Description

SUBSTANCE: invention relates to hydraulic engineering construction and can be used to create domestic and industrial waste accumulators.

A known method of creating a waste storage facility, including backfilling a drainage layer of soil, laying an impervious screen made of clay soil or polymeric materials, backfilling a protective layer of soil and monitoring the tightness of the screen (Manual for the design of landfills for the disposal and disposal of toxic industrial waste. CITP Gosstroy USSR. 1990, p. .22-24 analogue). The tightness control is carried out by filling the reservoir with water and observing the change in the level or inflow of water into the drainage system under the screen. When leaks are detected, water is pumped out of the storage tank, the protective layer is removed from the entire screen, and defects are searched for and eliminated.

It is possible to determine the location of defects in the screen during tests by the bubbles rising through the water layer when air is injected into the drainage layer under the screen (Manual for the design of landfills for the neutralization and disposal of toxic industrial waste. TsITP Gosstroy USSR. 1990 analogue). The method is characterized by a low accuracy in determining the location of defects. In addition, a conventional drainage system is not completely sealed, so by forcing air into it, it will not be possible to increase the pressure under the screen to values sufficient to force air through the screen and the water layer.

The problem of detecting defects can be partially solved by the method of controlling the anti-filtration efficiency of a two-layer screen, in which the drainage layer and the storage bowl are divided into sections by soil bridges, and filling with water, forcing air under the screen and eliminating the detected defects are carried out in sections (Certificate of CCCP No. 1564258 A1, IPC E02 B3/16, 1990 analogue). The disadvantage of this method is the large amount of work on the device and the subsequent removal of soil bridges and the difficulty of ensuring the tightness of each section of the drainage layer.

Screen continuity can be controlled by measuring the electrical conductivity of the protective and draining soil layers using electrodes placed over their entire area (Manual for the design of landfills for the neutralization and disposal of toxic industrial waste. TsITP Gosstroy USSR. 1990; Copyright certificate CCCP No. 1056332 A, MPK E02 B3 / 16, 1983 - analogues). There are also known methods based on monitoring soil moisture under the screen using a network of moisture sensors or electric cables with a semi-permeable sheath (Survey of technologies for monitoring containment liners and covers/ US Environmental Protection Agency/ Report 542-R-04-013. 2004. 44 p.analogue). These monitoring systems are expensive, since a dense network of electrodes or sensors is required to accurately determine the coordinates of defects.

When leaks are detected by one of the described methods, after removing the liquid, the protective layer is opened, the location of the defects in the screen is specified and they are eliminated, then the test is repeated and the protective layer is restored. To reduce the time for preparing the drive for waste acceptance, several methods have been proposed to restore the integrity of the screen during testing or operation of the drive.

A known method of embedding through defects in a film impervious screen of a soil hydraulic structure, including immersion through a liquid layer of a pressure device operating on the principle of a caisson, compaction of the soil layer underlying the screen, lifting the screen by reducing pressure in the pressure device and punching process holes in it, injection of impervious composition into the cavity under the screen (Author's certificate CCCP No. 1435685 A1, IPC E02 B3 / 16, 1988 analogue). When implementing the method, it is difficult to determine the exact location and size of the defect, as well as to ensure the tightness of the contact between the pressing device and the screen.

A known method of restoring the screen by injecting a sealing compound through tubular injectors previously placed in sections of the drainage layer separated from each other (US patent No. 4753551, 1988 equivalent). The implementation of the method is associated with the performance of labor-intensive operations for filling an additional drainage layer and installing a system of tubular injectors.

In a method for increasing the reliability of a liquid waste accumulator of industrial enterprises, a roll of polymeric material is delivered to the place of occurrence of a defect, the cloth is immersed on the bottom of a settling pond and loaded with waste in bags (RF Patent No. 2442857, IPC E02 B 7/06, 2012 analogue). The disadvantage of this method is the lack of reliable contact between the panel and the screen, since the surface of the screen is covered with a protective layer of soil and settled particles contained in the stored waste.

There is a known method of sealing the impervious screen under a reservoir after mining a quarry, in which cement mortar is injected through wells drilled from underground workings into the lower part of the screen at a pressure exceeding hydrostatic pressure, and after it hardens, it is plugged with local injections of synthetic resins (RF Patent No. 2551585, IPC E21 B 33/138, 2015 analogue). The method is characterized by high labor costs and cost, which limits its use only for sealing deep ore pits, and not for protecting the base of ordinary waste collectors.

Closest to the proposed invention is a method for creating a toxic waste storage tank on a highly deformable base, including backfilling of draining soil, laying and checking the tightness of a waterproof screen, dumping a protective layer, in which the leak test is performed by sequentially performing operations of scanning the base from the surface of the screen using physical measurements , feeding indicator liquid into the accumulator, its holding, subsequent pumping out and re-scanning of the base using physical measurements (RF Patent No. 2611167, IPC E02 B 3/16, 2017 prototype). As physical measurements, a magnetometric survey or activity measurement can be used, and as an indicator liquid, a ferrofluid or a solution of a compound of a short-lived radioactive element, respectively. The disadvantage of this method is the cost of the operations of double scanning of the base before and after the indicator liquid is fed into the drive, the elimination of defects in the screen, and the repeated supply of the indicator liquid.

The objective of the invention is to increase the operational reliability of the drive by ensuring the integrity of the waterproof screen, as well as to reduce the time for putting the drive into operation by combining the operations of checking the tightness of the screen and eliminating existing defects.

This is achieved by the fact that in the method of creating a waste accumulator, including preparation of the base by backfilling the draining soil, laying a waterproof screen, filling the protective layer of soil, supplying the fluid to the accumulator and subsequent pumping out to check the tightness of the screen, a suspension of fine clay is used as a liquid, in the draining soil and soil of the protective layer during filling, and a flocculant is introduced into the liquid before pumping. Saponite-containing kimberlite ore enrichment waste can be used as a suspension of fine clay, chalk as a flocculant introduced into soils, and Portland cement in an amount of 0.41.2% by weight of the suspension as a flocculant introduced into a suspension of fine clay.

The method is illustrated by drawings, where figure 1 shows the design of the drive bed after testing in the absence of a defect in the screen, figure 2 is the same in the presence of a defect. The dynamics of sedimentation and the appearance of suspension samples after settling for 172 minutes are shown in Fig.3 and Fig.4, respectively, where the numbers indicate the numbers of experiments.

The method is carried out as follows.

After the embankment dam is erected along the contour of the reservoir, on the soil base 1, the draining soil 2 is filled and compacted with the addition of a flocculant, then a waterproof screen 3 is laid from local clay or a polymer material, usually called a geomembrane. Having completed the laying of the protective layer of soil 4 with the addition of a flocculant, they proceed to check the tightness of the screen. A pre-prepared suspension of fine clay 5 is fed into the accumulator bowl and a technological break is arranged.

If the level of the suspension in the accumulator remains constant and there is no inflow into the drainage layer, a flocculant is added to the suspension. Under the action of the flocculant, aggregates of clay particles are formed, which, settling in the reservoir bowl, form a waterproof coating 6 on the surface of the protective layer, which serves as the second additional layer of base isolation from the penetration of the filtrate from the reservoir. Clarified water is pumped out of the storage tank.

In the presence of defects in the screen, for example, sand lenses 7 in the clay screen or holes in the geomembrane, the suspension 5 is filtered through the protective layer of soil 4, lens or hole and penetrates into the layer of draining soil 2. Clogging of the pores by particles of fine clay leads to the formation of above and below screen zones of limited dimensions in terms of 8 and 9, respectively, with low water permeability. The effect of pore clogging is enhanced and accelerated due to the effect of a flocculant introduced into the soil in advance, which causes aggregation of fine clay particles. Thus, the defect in the screen is plugged. While maintaining a constant level of the suspension and the absence of inflow into the drainage layer, a flocculant is introduced into the suspension. On the surface of the protective layer, the settling particles form a waterproof coating 6. Clarified water is pumped out of the accumulator, thereby completing the test. If necessary, the suspension can be additionally fed into the accumulator, maintaining a predetermined pressure.

After successfully carrying out the combined operations of checking the tightness of the screen and eliminating existing defects, the drive is put into operation.

It is known that sand with the addition of 15% or more fine clay (bentonite) has almost the same filtration coefficient as fine clay itself, and a mixture with 5-15% clay is used to isolate reservoirs (R.P. Chapuis. The 2000 R.M. Hardy Lecture: Full-scale hydraulic performance of soil-bentonite and compacted clay liners, Canadian Geotech J. 2002, No. 39, pp. 417-439 https://cdnsciencepub.com/doi/10.1139/t01-092). According to A.F. Ghazy, the optimal content of bentonite in the preparation of mixtures with sands of various granulometric composition is 5-10% (A.F. Ghazi. Engineering characteristics of compacted sand-bentonite mixtures/M. thesis. Edith Cowan Univ. 2015. 84 p.https://ro.ecu .edu.au/theses/1615). The minimum content of bentonite in an artificial mixture for use in landfills is considered to be 1.5% (LFE10 Using bentonite enriched soils in landfill engineering. UK Environment Agency. 24 June 2014/ 51 p.https://www.gov.uk /government/publications/using-bentonite-enriched-soils-in-landfill-engineering-lfe10).

The level of the suspension in the reservoir when checking the tightness should be determined from the condition of ensuring such a rate of its filtration through the protective layer and the layer of draining soil so that fine clay particles are not carried into the base. The indicated speed depends, in addition to the level of the suspension, on the particle size distribution and density of the soil in the indicated layers, the concentration of the suspension, etc. This speed can be determined experimentally, for example, in filtration devices. The concentration and type of flocculant must also be determined experimentally.

In addition to the suspension of bentonite, liquid industrial waste from the mining industry can be used in the implementation of the method. In particular, at the enterprises of the diamond mining industry of the Arkhangelsk region, technogenic soils, tailings for the enrichment of kimberlite ore, containing saponite, a highly dispersed clay mineral, are not used. The suspension of the clay fraction, which has a high physical and chemical activity, occupies significant volumes of the tailings, and its clarification is extremely slow. Therefore, as a suspension of fine clay, kimberlite ore enrichment waste from the tailings pond-sump can be used.

Below are the results of experiments that confirmed the possibility of implementing the method. A saponite-containing suspension was filtered through samples of fine sand from the tailings pond-settlement pond.

As a flocculant introduced into the soil, widely used substances can be used, for example, aluminum sulfate, ferrous sulfate, ferric chloride, etc. In our experiments, we used chalk, which does not pose a danger of groundwater pollution. It is known that calcium ions contribute to the aggregation of clay particles (Ground science / Under the editorship of E.M. Sergeev M.: Publishing house of Moscow University, 1983. 392 p.).

Example 1. Tests were carried out on samples of fine sand with a diameter of 73 mm and a height of 145 mm at two values of the compaction coefficient of 0.900.92 and 0.930.96, which are characteristic of the drainage and protective layers of the storage bed. The concentration of the suspension taken from the settling pond of the tailing dump was 0.580.63%. The solid phase was represented mainly by particles smaller than 0.001 mm.

The first series of experiments was carried out on sand samples, the second on sand samples with the addition of chalk (CaCO ₃ ) in an amount of 3 and 5% by weight of dry sand. The chalk was pre-crushed and sieved on a sieve with a hole size of 0.1 mm.

After saturating the samples with water, the initial value of the filtration coefficient was determined. Then, a suspension was passed through the samples at a rate of 3.5 m/day or more, at which clay particles do not settle on a porous disk placed on the upper end of the sample. The specified speed was found empirically. The suspension was passed for 1 hour with a gradually increasing pressure gradient. The tests were completed by re-filtering the water. Each series of experiments was repeated two or three times.

The results of the experiments are shown in Table 1, where it is indicated: K _com coefficient of compaction of the samples, k ₀ - initial filtration coefficient, k _m filtration coefficient after passing the suspension, n ratio k ₀ /k _m .

Table 1. Influence of pore clogging by clay particles on the water permeability of sand samples Priming _Kcom k ₀ , m/day k _m , m/day n Sand 0.90…0.92 2.05 0.99 2.1 0.93…0.95 1.85 0.96 1.9 Sand,
3% _CaCO3 0.91…0.92 1.58 0.05 29.9 0.94…0.95 1.30 0.08 15.3 Sand,
5% _CaCO3 0.91…0.92 1.11 0.03 38.5 0.93…0.95 0.67 0.03 25.9

Experiments with sand samples showed a decrease in the filtration coefficient after passing the suspension by only 1.85 and 2.05 times. The reason for such a small effect was the removal of clay particles from the samples by the water flow even at a pressure gradient of 0.25.

The presence of a flocculant led to an insignificant decrease in the water permeability of sand, so the filtration coefficient of samples with the addition of 3 and 5% chalk turned out to be 1.31.4 and 1.82.8 times less than that of samples without additives, respectively. When filtering the suspension, the removal of clay particles from the samples was not observed. Clogging of pores with clay particles led to a decrease in water permeability - by 15.338.5 times. With an increase in the duration of the passage of the suspension through the samples, the effect of clogging will increase.

Thus, the experiments have shown that saponite-containing kimberlite ore enrichment waste can be used as a suspension of fine clay, and chalk introduced into the soil as a flocculant provides reliable clogging of pores during suspension filtration.

Example 2. Tests for the sedimentation of fine clay particles were carried out on suspension samples taken from the tailing pond. The suspension concentration ranged from 0.24 to 0.67% (2.4 - 6.7 g/l). The sample volume was 25 ml. Portland cement was used as a flocculant in an amount of 0.4 to 1.2% by weight of the suspension. The duration of observations was 172 minutes.

The results shown in table 2, figure 3 and figure 4, indicate the effectiveness of the use of this additive - the layer of water over the suspension at the end of the experiments was transparent and colorless. In the control experiment, where the concentration of clay particles in the suspension was 0.67%, the sedimentation of clay particles during the observation period was extremely slow (number VI in figure 4). Note that the introduction of cement provides not only rapid clarification of water, but also the formation of structural bonds between particles in layer 6.

Table 2. The volume of clarified water (ml) 172 minutes after adding Portland cement to the suspension Additive
Portland cement
in % by weight
suspensions Suspension concentration, % 0.24 0.51 0.67 0.4 22.0(I) - 19.4(II) 0.9 - 20.8(v) - 1.2 22.4 (III) - 19.2(IV)

Note: Numbers of experiments are given in parentheses.

The proposed method makes it possible to increase the operational reliability of the waste accumulator by ensuring the integrity of the waterproof screen, as well as to reduce the time for putting the accumulator into operation by combining the operations of checking the tightness of the screen and eliminating existing defects.

Claims (2)

1. A method for creating a waste storage tank, including preparing the base by backfilling the draining soil, laying a waterproof screen, backfilling the protective layer of soil, supplying it to the storage tank and then pumping out liquid to check the tightness of the screen, characterized in that the draining soil and soil of the protective layer during backfilling flocculants are introduced, and a suspension of highly dispersed clay is used as a liquid, into which Portland cement is added after it is fed into the accumulator in an amount of 0.4-1.2% by weight of the suspension, after which the clarified water is pumped out of the accumulator. 2. The method according to claim 1, characterized in that saponite-containing kimberlite ore enrichment waste is used as a suspension of fine clay.

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