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Abstract

Content

Introduction

The scale of coal pitch production is determined primarily by the production of coal tar, and its volume depends on the amount of metallurgical coke consumed in the production of cast iron. Coal tar obtained at coke plants is the raw material for the production of electrode pitch. The most active consumers of electrode pitches are non-ferrous metallurgy (aluminum production) and the actual production of electrodes for various purposes by electrode and aluminum plants (including for steel smelting in electric arc furnaces). According to the level of consumption, the first direction should be considered the main one. For example, in Russia, the aluminum industry accounts for up to 88% of the electrode pitch, or up to 52% of the total pitch consumed (including the production of pitch coke, refractories, prepared resins, etc.).

The initial pakes produced using currently existing coal tar distillation technologies cannot be directly used as electrode materials. The production technology of the electrode pitch should ensure the possibility of achieving optimal values of a set of quality indicators responsible for both the binding and coke-forming properties of the product.

1. Theme urgency

Coal pitch is the most valuable product of coal tar processing, used, among other things, for the production of electrode products as a binder. Despite the fact that research on the development of alternative electrode binders (in particular, oil-based ones) has been actively conducted in a number of countries for a long time, there is currently no equivalent replacement for coal-fired pitch in this area. This determines the constant stable demand for coal electrode furnaces.

The master's thesis is devoted to the urgent task of controlling emissions during the processing of coal pitch, such as coke dust, coal dust and gaseous substances.

2. Goal and tasks of the research

The purpose of the study is a feasibility study and study of the processes of obtaining high-temperature pitch at coke chemical plants.

Main tasks of the research:

  1. Analysis of the problem of obtaining high-temperature pitch using an electrode binder.
  2. Study of methods for solving existing problems of obtaining high-temperature fuel.
  3. Study of the impact on the atmosphere of emissions from coke plants.
  4. Search for optimal ways to reduce emissions from the production of coal pitch.
  5. Development of alternative options for processing industrial waste.

Research object: coal pitch.

Research subject: the process of obtaining high-temperature pitch using an electrode binder.

3. Review of research and development

Despite the ongoing research on the creation of a synthetic electrode binder with specified properties, coal pitch is actually widely used, the scale of production and consumer properties of which are directly related to the production of metallurgical coke. Modern world trends in the modernization of ferrous metallurgy are aimed at reducing the consumption, and hence the production of metallurgical coke, due to the development of a competing process for producing steel in electric arc furnaces and increasing the number of blast furnaces with pulverized coal injection. Nevertheless, the global production of coke is quite large and amounts to ~ 350 million tons/year. With this volume of production, more than 15 million tons/year of coal tar is formed as a by-product. Although most of it, in one form or another, is burned, it is quite enough to meet the global demand for electrode coal pitch (about 3.3 million tons/year). But since producers and consumers of coal pitches are often separated by long distances, there is currently a shortage of electrode binder in some regions [2].

3.1 Review of international sources

The most important raw material component in the production of most types of carbon products are binding materials, the quality of which largely determines the level of physical and mechanical properties of the product [1]. The favorable combination of high coke-forming ability and low viscosity in the molten state favorably distinguishes coal pitch from other types of binding materials for various carbon-containing compositions that are subsequently subjected to carbonation. For this reason, electrode coal pitch is a priority binder all over the world for the production of many types of carbon products (anode masses, electrodes, structural materials, electric coal products, etc.) [2].

In most regions of the world, relatively small amounts of electrode pitch are produced, which does not always satisfy its own needs. The greatest deficit is observed in countries with a developed aluminum industry, which is the dominant consumer of coal pitches. First of all, this applies to the countries of North America and Russia, where the shortage of pitch is compensated by imports [2].

3.2 Overview of national sources

Currently, the most common method in the Russian industry for producing medium-temperature coal pitch by single evaporation of resin preheated in a tubular furnace to 360-420 °C practically does not allow to regulate the content of alpha- (substances insoluble in toluene) and alpha-1-fractions (substances, insoluble in quinoline) in pitch, therefore, these pitch indicators are almost completely determined by the properties of the resin coming for processing. In turn, when the density of the resin entering the processing changes, difficulties arise with obtaining the pitch of the required quality, especially in terms of the content of α- and α1-fractions. Separately, it should be noted that the content of the α-fraction is almost inversely proportional to the yield of volatile substances. That is, by increasing the content of substances insoluble in toluene, thereby reducing the yield of volatile substances. This effect becomes more relevant when resin with a low degree of pyrolysis is available as a raw material.

Special attention is required by the proposed methods for improving the quality of pitch by affecting the raw material – coal tar. The most significant factors affecting the content of α-fractions in the baking are the temperature and consumption of raw materials [3].

The previously widely used method of blowing volatile substances out of pitch with air oxygen, today, is becoming less common for a number of reasons. The issues of mixing in a pitch reactor without air have become more topical and provide an opportunity for further research. Also, one of the promising directions in improving the quality of pitch is to increase the residence time of pitch in the equipment. This method is implemented by using a cascade of reactors [4].

4. Overview of the coal pitch processing technology at the enterprise CJSC "MAKEEVCOKS"

In order to reduce the burden on the environment, the least expensive way is to modernize the existing technology in order to reorient it to work in new conditions. At CJSC MAKEYEVCOX, the company investigated the possibility of adjusting the yield and properties of medium-temperature pitch (STP) by introducing a sulfur-containing additive of coal origin into the coal tar before the second stage of heating. However, this method, as well as, for example, the addition of ammonium sulfate to the STP during heat treatment, despite its effectiveness, has its limitations. The limiting factor in this case is the mass fraction of sulfur in the coal electrode cake. For example, in technical conditions, at the request of consumers, in contrast to GOST 10200-83, the mass fraction of total sulfur (α) is strictly normalized. Since this indicator depends on the similar characteristics of the feedstock and on the amount of sulfur-containing additives, in some cases the use of such techniques can be risky.

In order to maintain the proper level of quality indicators of the electrode pitch in conditions of a decrease in the degree of pyrolysis of raw materials, the acquisition of highly pyrolyzed resin from the side for compounding with available low-pyrolyzed resin was considered as one of the methods. Based on the above recommendations, a technology for averaging imported resins and resins of own production has been developed [5].

In the resin distillery of the MAKEYEVCOX enterprise, the most common installation in the post-Soviet countries for the production of electrode binder by thermal oxidation of medium-temperature pitch (STP) obtained at the stage of single evaporation of coal tar in the distillation department functions [6]. The essence of the method is the heat treatment of STP at 340-370 ° C in continuous-acting reactor cubes with continuous bubbling of air through the processed melt. Air oxygen is a factor that intensifies compaction reactions (polycondensation and polymerization), which lead to the necessary changes in the physicochemical properties of pitch. The temperature of the oxidation process is maintained due to the temperature of the STP at the outlet of the evaporator (to a greater extent) and the exothermic effect of oxidative reactions (to a lesser extent). The advantages of this technology, which have become the reason for its widespread use, are the simplicity of hardware design and control of the oxidation process.

5. Reduction of gaseous emissions generated during the processing of high-temperature pitch

The most important direction of reducing industrial emissions into the air basin is to improve the production technology of processes and basic technological equipment. When choosing technological units, preference should be given to more powerful units [7]. For example, a blast furnace with a volume of 5000 m3 replaces an entire blast furnace shop and only by reducing the sources of dust and gas emissions, dust and carbon monoxide emissions are significantly reduced.

Replacing fuel in metallurgical units with electricity significantly reduces dust and harmful gas emissions. The elimination of unnecessary operations and intermediates associated with dust and gas emission can contribute to a significant reduction in emissions into the atmosphere. The transition from periodic to continuous processes makes it possible to greatly reduce dust and gas emissions. For example, the transition in blast furnace shops from skip feeding of materials to conveyor reduces dust emission several times. Equipping technological units with anti-dust devices significantly reduces the release of dust into the atmosphere. An example of such devices can be devices for smokeless loading of coke ovens and multi-layer oxygen tuyeres.

Work on conditioned raw materials corresponding to technical conditions also contributes to reducing the amount of emissions.

When carrying out technological processes in closed volumes, as is the case in various furnaces or steam boilers, the bulk of dust and gas emissions are removed in an organized manner through exhaust ducts and chimneys. In conditions when a particular process is open, an important place in the fight against air pollution is occupied by the prevention of dust and gas emissions by suppressing them in places of formation. Depending on the specific conditions of the process, the suppression of dust and gas emissions can be carried out in various ways [8]. Of great importance is the process of coking, door sealing, the integrity of which requires constant monitoring.

In general, the following methods can be used to purify gases from chemical gaseous impurities:
a) Absorption is the process of separation of gas mixtures using liquid absorbers - absorbers. If the absorbed gas (absorbent) does not chemically interact with the absorbent, then the absorption is called physical (the non-absorbed component of the gas mixture is called inertia, or inert gas). If the absorbent forms a chemical compound with the absorbent, the process is called chemisorption [9].
b) Adsorption - absorption of gas impurities by solids, for example, activated carbons [10].
c) Conversion of gaseous impurities with the help of special additives into a solid or liquid state, followed by their release from the gas.

Conclusions

The methods of additional heat treatment of electrode pitches are based on various mechanisms of formation of high-molecular fractions that determine the coke-forming and rheological properties of pitch.

The master's thesis is devoted to the urgent task of substantiating schemes for obtaining high-temperature pitch of coke chemical enterprises. Based on the analysis of literary sources, the following conclusions have been made:

    It is shown that coal pitch is the main determining raw material for the production of electrode pitch that meets the requirements of various consumers.
  1. Basing on the introduction of the technology of thermal oxidation of low-pyrolyzed resin to obtain a pitch of a given quality, including the stages of thermal oxidation by air and thermal support of oxidized pitch.
  2. The choice of thermo-oxidizing technology for the production of electrode pitch at the resin processing plant of CJSC MAKEVCOX has been studied and substantiated.
  3. The increased reactivity of low-pyrolyzed coal tar in thermal and thermo-oxidative processes is shown. Obtaining an electrode pitch from resins of various degrees of pyrolysis required adjustment of the technological regime of the oxidation stage.
  4. A reasonable way to reduce gaseous emissions of low-temperature pyrolysis processes has been substantiated.

This master's work is not completed yet. Final completion: May 2023. The full text of the work and materials on the topic can be obtained from the author or his head after this date.

References

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