PROCESSING AND DISPOSAL OF SOLID WASTE, Mechanical processing...

CHAPTER 17 PROCESSING AND DISPOSAL OF SOLID WASTE

In the practice of recuperation of solid industrial waste, methods for enriching the processed materials are used: gravity, magnetic, electrical, flotation and special. Many processes of solid waste utilization are based on the use of the method of leaching (extraction), dissolution and crystallization of processed materials.

In the utilization and processing of solid waste, various methods for the thermal treatment of raw solid materials and products are used: pyrolysis, remelting, roasting and fire detoxification (incineration) of many solid waste types on an organic basis.

17.1. Mechanical processing of solid waste

The variety of solid waste types, the significant difference in the composition of waste with the same name, complicates the tasks of their utilization. At the same time, various technologies for recovery of solid waste are basically based on methods, the combination of which provides the possibility of recycling secondary material resources or processing them into the target products.

The most rational way to protect the lithosphere from waste production and life is to develop special technologies for the collection and processing of waste.

Processes such as crushing and grinding, classification and sorting, enrichment in heavy media, jigging, magnetic and electrical separation, drying and granulation, thermochemical calcination, extraction, etc. are used for solid waste processing

The intensity and efficiency of most chemical and biochemical processes increases with the reduction in the size of pieces of processed materials. In connection with this, the technological operations of processing solid waste are usually preceded by operations to reduce the size of their pieces.

For those industrial wastes, the utilization of which is not related to the need for carrying out phase transformations or the action of chemical reagents, but which can not be used directly, two types of machining are used: grinding and compacting (pressing). This applies equally to wastes of both organic and inorganic origin. After grinding, which can be followed by fractionation, the waste is converted into products ready for further use. The solid material can be broken down and crushed to the desired size by crushing, splitting, breaking, cutting, sawing, abrasion and various combinations of these methods.

Crushing. Milling is the process of multiple destruction of a solid body under the influence of external loads exceeding the forces of molecular attraction in the grinding body. The grinding process is accompanied by a multiple increase in the specific surface area of ​​the material to be crushed, which makes it possible to intensify sharply the chemical and mass-exchange processes whose velocity is determined by the area of ​​the interfacial interaction.

The processes of grinding are widely used in the technology of solid waste recycling during the processing of dumps of minerals, building structures and products, some types of mixed scrap of ferrous and non-ferrous metals, fuel and metallurgical slag, coal waste, some industrial sludges and waste plastics, pyritic cinders and a number of other secondary material resources.

The grinding process is characterized by the degree of grinding i - the ratio of the average size of the original solid waste d H to grinding to the average size of the material after grinding d K :

(17.1)

Since the source and particulate material are polydisperse composition for evaluation use different characteristics: the largest d max and the smallest d min particle diameters; varying the magnitude R = d max /d min ; average particle diameter d cp ; granulometric (dispersed) composition, characterizing the fraction or percentage of the mass of particles of each class of size; Specific surface area of ​​particles S.

Particles of a solid material have an irregular shape, therefore, by their size is meant the diameter of a sphere equivalent in volume:

(17.2)

or specific surface

(17.3)

where V is the volume of the particle; S = S sex /V - specific surface of the particle; S sex is the total surface of the particle.

The distribution functions D (x) and R (x) are used in the polydisperse bulk material to describe its particle size distribution, equal to the ratio of the mass of the particles, whose size is correspondingly smaller and larger than d, to their total mass. In this case, the equality

(17.4)

Among the various laws describing analytically the distribution functions of particles of bulk material for one or another characteristic, the most common laws are Rosina-Ramler

(17.5)

and a normal-logarithmic

(17.6)

where x e is the particle diameter at which the mass of particles larger than x e is 36.8%, and less - 63.2%; n is a parameter characterizing the uniformity of the material by size; fix) is the frequency of the values ​​of x; N is the total number of observations x; - the average particle diameter in the sample; σ is the standard deviation of the values ​​of x from x.

The degree of uneven granulometric composition is characterized by the coefficient of variation

Depending on the size of the initial and pulverized solid material, the processes of crushing and grinding are distinguished. By crushing is understood the process of reduction in size, as a result of which the maximum size of a piece in the crushed material is equal to or more than 5 mm. Under the grind is meant the process of reducing the size, as a result of which the maximum grain size in the crushed material is less than 5 mm. These processes, depending on the size of the pieces of the initial material and the final size of the resulting material, are conditionally divided into several classes (Table 17.1).

Table 17.1. Classes of grinding

Class

Size of pieces, mm

Checkout

Size of pieces, mm

before grinding

after grinding

before grinding

after grinding

Crushing

Milling

large

& gt; 500

100 ... 300

large

20 ... 100

1 ... 4

mean

100 ... 500

20 ... 100

Medium

5 ... 50

0.1 ... 1

shallow

50 ... 100

4 ... 20

Thin

1 ... 10

0.01 ... 0.1

hyperfine

0.1 ... 1

0.01

The grinding ratio for large grinding is i = 3 ... 8, for fine and fine grinding - up to i = 100. Crushing of solid materials is usually carried out in a dry way, and fine grinding - in a wet process, which eliminates dust formation.

The main methods of grinding are impact, crushing, abrasion, rolling, cutting.

When impact (Fig. 17, a, b) under the influence of dynamic loads in the material, dynamic stresses arise, leading to its destruction. Distinguish loose and cramped blows. If the impact is hampered, the material collapses between the two working organs of the chopper, and when it is free, it collides with the working body or other chopped body.

When crushing (Figure 17.1, c) under the action of a static load, the compressive stresses are the determining ones.

When abrasion occurs (Figure 17.1, d), the failure occurs from shear stresses. Rubbing in combination with crushing is one of the most economical ways of grinding.

Fig. 17.1. Methods of grinding solid bodies: a - cramped blow; b - free kick; c - crushing; d - abrasion; d - splitting; e - cutting; p is the load; ν is the speed

When splitting (Figure 17.1, d), bending stresses arise in the material.

Cutting is accompanied by the appearance of shear stresses in the material.

Work A, spent in grinding to break the initial material, is directly proportional to the newly formed surface S:

(17.7)

where k 1 - coefficient of proportionality; ΔS is the increment of the surface.

The work of internal elastic forces in the absence of loss is equal to the work of external forces that caused elastic deformation of the body:

(17.8)

where σ is the stress arising during deformation; V is the volume of the deformed body; E is the modulus of elasticity (Young's modulus). The work of grinding a single piece of size D is equal to

(17.9)

where k 2 is the proportionality coefficient.

In the generalized form, the work spent on deformation of the pieces to be broken and the formation of new surfaces is

(17.10)

where γ, σ are proportionality coefficients; ΔV - deformed volume; ΔS is the newly formed surface.

In pure form, the work during crushing is proportional to the geometric mean between the volume V and the newly exposed (formed) surface S:

(17.11)

where k B is the Bond coefficient.

Machines for crushing and grinding solid waste on a mineral basis are machines using grinding methods based on crushing, splitting, breaking, abrasion and impact. Crushing of solid waste on an organic basis is carried out in machines, the principle of operation of which is based on sawing, cutting and impact.

In addition to crushing by mechanical means, special methods based on various physical phenomena, in particular, destruction of materials by means of electrohydraulic effect, compressed medium, decryption, etc., are used.

The electro-hydraulic effect is based on the use of a high-voltage discharge in a liquid. The significant thermal power released during the discharge leads to heating of the substance to tens of thousands of degrees, its evaporation and ionization. The discharge products behave like gaseous explosion products, which causes the appearance of ultrahigh hydraulic shock waves, cavitation, ultrasonic radiation, resonant effects, destroying the material. The sources of electrical discharge are current pulse generators with capacitive energy storage.

At present, the electro-hydraulic effect is used in metalworking (forming of tubular and hollow articles, parts from low-plastic materials), mining (drilling, crushing and grinding), agriculture, food industry, chemical technology, diamond mining and other industries industry.

Destruction by a compressed medium (explosion) is the creation of excessive pressure in pieces of crushed material, their subsequent exposure to it and its sudden release. The explosive method is used to destroy materials such as coal, asbestos, ore, wood.

At high pressure, the gaseous medium (vapor, air) penetrates into the pores and cracks of the lump, already at this stage the weakened material due to adsorption processes in the pores, cracks, planes of fusion of minerals and the formation of microcracks in weaker areas. With subsequent rapid release of pressure, the gas, expanding, destroys the material.

One of the drawbacks that arise when grinding viscous, elastic and viscoelastic materials (rubber, some types of thermoplastics, etc.) is that at room temperature the energy consumption for their processing is very high, although not more than 1 % of energy, the main part of it is converted into heat. Therefore, in the last 15 ... 20 years, a cryogenic grinding technique has been increasingly used, which allows the material to cool below the brittle temperature. As a rule, liquid nitrogen, having a temperature of 196 ° C, is used as the cooling agent, which is below the brittleness temperature of most polymer materials.

With this method of crushing, the degree of grinding sharply increases, the productivity of the process increases, the specific energy consumption decreases, the oxidation of the product is prevented.

Sieving. When crushing solid waste, the degree of grinding of materials is different. It depends on the hardness, fragility and the original shape of the piece. After each stage of crushing, a part of the material may be smaller than the given size and will be an extra load for the next crushing machine, therefore, before the crushing and between the remaining stages, the material is sorted into classes using screening devices.

To separate lumpy and loose materials into fractions, various methods are used: sifting or screening; separation under the action of gravitational-inertial forces; separation under the action of gravitational-centrifugal forces.

Screening is the process of dividing into classes by the size of differently sized pieces (grains) of material when it moves on cellular surfaces. Separation into fractions is carried out by using various designs of screens, gratings and screens.

In the second and third cases, the separation of crushed products into classes or the isolation of the target product is carried out by the method of separate precipitation of particles from the carrier medium under the action of gravitational-inertial or gravitational-centrifugal forces. As a carrier medium with dry grinding, air, less often smoke or inert gases, and with wet water is used most often.

The operation of the screen is characterized by an efficiency coefficient representing the ratio of the weight of the separated lower class to its mass in the raw material fed to the screen. The material that does not pass through the holes of the screen is called the upper class (the oversize product) and is indicated by the plus sign. Accordingly, the material that has passed through the holes of the screen is called the lower class (sublattice product, sifting) and is denoted by a minus sign. Screening is usually used to separate products with a grain size of 1 mm or more, although there are cases when it is used to isolate thinner classes (up to 0.06 mm).

It is unsuitable for thin (shredded) materials, as they aggregate (crumple), reducing the efficiency factor of the screen, easily sprayed. These materials are divided by their size in air (air separation) or in aqueous (hydraulic classification) media using appropriate apparatus.

Hydraulic classification has become very widespread in the enrichment of ores of ferrous and non-ferrous metals, in the chemical industry. Like air separators, hydraulic classifiers are divided into gravitational and centrifugal by the nature of the acting forces.

Pressing and compacted waste. Mechanical pressing and compacting of solid waste (industrial and domestic, organic and inorganic) is one of the main methods of reducing their volume for the purpose of more rational use of road and rail transport, transporting waste to the places of their utilization and storage.

Pre-compaction processes to increase the productivity of the grinding stage sometimes need to be applied to those wastes that have a low bulk density (for example, foam waste, film trimming, etc.). Pressing at high pressures is one of the ways to improve the conditions of operation of landfills (dumps). The compacted waste gives a smaller amount of filtrate and gas emissions, while the probability of fires is reduced, the land area of ​​the landfill is used more efficiently.

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