Zinc and its impact on the environment. Production and use of zinc and its compounds

It can be considered conditionally environmentally friendly. Conditionally - because there is none industrial production, which would not be thrown into environment substances that are not utilized by nature. Indeed, in this production there are also emissions of gases - both from burners (NOx) and from surface preparation baths (volatile acids). Accidental spills of surface preparation agents are possible, typically resulting in the release of these agents into either storm sewers or municipal wastewater.

Nevertheless, this production is ready to ensure that these emissions do not exceed the maximum permissible parameters, and the planned production waste is either disposed of to obtain useful products, or converted into non-toxic sludge suitable for storage in landfills with minimal environmental damage.

The main production wastes at the stage of dipping products into the melt are ash, hardzinc and flying dust, which is formed when products are dipped into the melt as a result of flux decomposition.

The easiest way to recycle the first two components is to sell them. The fact is that ash contains up to 90%, and hardzinc contains about 96% of metallic zinc. Until recently, a fairly large consumer of zinc and its compounds was the paint and varnish industry, the production of zinc white. However, at present, zinc white is being intensively replaced from production by other types of white paints, mainly based on titanium dioxide.

Another way to dispose of this waste is to send it to zinc metal plants. Abroad, there is a certain fee for these wastes - approximately 30 and 70% of the price of metallic zinc, respectively. However, this method of disposal is acceptable for Western Europe (due to the small transportation distance), somewhat less acceptable for the New World, and probably unacceptable for Russia due to the small mass of the transported cargo and large distances.

But even in the conditions of Western Europe, people are thinking about how to reduce the loss of zinc, to return at least part of it back to the process. For this, they devise various ways, but they are all based on the physical state of the waste. Consider each type of waste separately.

Zolka

Appearance this waste is shown on rice. 13.19. Cindering occurs as a result of the fact that the workers free the surface of the bath with scrapers before immersing the products in the melt from the oxide film. In this case, the scraper captures not only zinc oxide (actually ash), but also metallic zinc. This metallic zinc is present in the ash in the form of small droplets ranging in size from fractions of a millimeter to several millimeters.

The possibility of the process of returning a significant part of metallic zinc by the following operations was studied: grinding the ash, sifting into separate fractions and returning part of the fractions to the bath with the melt. It is shown that this method is cost-effective.

It is also proposed to slightly change the conditions for raking ash. The ash is not raked outside the bath, but into a special device (ash collector) located at one end of the bath above the melt. Under these conditions, particles of metallic zinc, being under the temperature influence of the melt, flow back into the melt, as a result of which the chemical composition of the ash approaches more and more zinc oxide (64% zinc in the compound).

Rice. 13.19. Appearance of ash - one of the main recyclable wastes of the hot dip galvanizing process.

It is also possible to periodically spill ash on the surface of the bath when the latter is inoperative; the same processes take place as in the previous case.

The disadvantage of these methods is the presence of an oxide layer on the surface of metal zinc droplets, which does not melt at the bath temperature, and therefore prevents the melted particles from quickly sticking together into larger aggregates. Therefore, special mixtures of the flux-zinc type have been proposed, small additions of which significantly speed up the processing of ash and make it possible to return up to 80% of the metallic zinc in the ash to the bath. The chemical composition of the mixtures is not reported, but it can be easily determined chemically. As a rule, it is ammonium chloride or nitrate, to which some substances have been added to prevent their caking and spontaneous explosiveness.

Plants are also available for mechanically separating zinc droplets from zinc oxide. According to these methods, the technical product is ground and then sieved into fractions. Zinc oxide, as a more brittle product, turns into a powder fraction, and larger particles are metallic zinc.

Recently, in our opinion, a more progressive method of processing ash in specially designed furnaces has been widely advertised ( rice. 13.20). It has been experimentally shown that with general galvanizing, the ash contains up to 70% of metallic zinc, and with galvanizing pipes and galvanizing using centrifuges - up to 80% zinc. At the same time, it has been shown that up to 90% of metallic zinc can be recovered from this waste. Practice confirms these conclusions. For the operation of such a furnace, 10 cubic meters are required. m of combustible gas per hour, the furnace is serviced by one worker, and the cost of reclaimed zinc is $0.125/kg, which is cheaper than purchased by more than an order of magnitude.

Hartzink (dross)

Hardzinc is an iron-zinc alloy of the approximate formula FeZn 7 with a density of about 7, with a melting point of about 530°C. The ways of its formation are obvious - this is the interaction of molten zinc with iron, which is part of the flux dried on the surface of the product; it is also the interaction of zinc with iron leaving the surface of the product during the formation of an iron-zinc alloy; this is also the result of the interaction of zinc with the walls of the bath. The resulting hardzinc, being heavier than the zinc melt, settles to the bottom of the bath. In order to more effectively remove hardzinc from the bottom of the bath, a lead pad is placed on its bottom. In this case, the removal of hardzinc with a grapple is quite effective. In addition, there is no accretion of hardzinc to the bottom of the bath.

The available manuals do not focus the reader's attention on the fact that hartzinc crystals, which are born small, continuously grow, and only after reaching a certain size, they begin to sink to the bottom. Different baths, being equipped with burners of various designs, create different flows of liquid in the baths. As a result, at the bottom of the baths differing in the method of heating, a deposit of hardzinc of different sizes is formed.

The hardzinc extracted from the bath, being poured into ingots for subsequent disposal, outward signs not much different from ingots of metallic zinc ( rice. 13.21 ). If you look at its section under a microscope, you can see ( rice. 13.22) that particles of hartzinc "float" in zinc, that is, together with hartzinc, pure zinc also goes to waste. Since the waste in the form of hardzinc is a good starting material for obtaining again metal zinc from it, buyers pay real money for this waste. Usually they indicate that they accept hardzinc for processing with a concentration of the target product (zinc) of 92-96%. Let us calculate how much excess zinc, which is still capable of serving technological purposes, is given at the price of waste.

Rice. 13.20. Appearance of the device for processing ash at the site of galvanizing production and obtaining the final product.

Calculations show that pure hardzinc of the formula FeZn 7 should contain 11.1% iron. In real waste, the iron content ranges from 4 to 8%. Elementary calculations show that when the iron content in hardzinc is 4 and 8%, the proportion of hardzinc in ingots is 36% and 72%, respectively, that is, the content of pure zinc is 64% and 28%.

The latter value is, in principle, equal to the fraction of voids between the close packing of hardzinc crystals, if the latter are taken as spherical particles, and a better result can be achieved only with the help of heat treatment waste (more on that later). However, a value of 64% suggests that at least half of the waste zinc could be saved for production. Firstly, it is necessary to take measures to increase the completeness of hardzinc precipitation in the bath - they are carried out special events– bubbling of the melt.

There are patent solutions that propose heating hardzinc in a special crucible (similar to the aforementioned ash solution), as a result of which hardzinc crystals settle to the bottom of the crucible, and metallic zinc is on top, which can be returned to the bath. It is not known, however, whether the economic side of the issue has been worked out.

Rice. 13.21.

Rice. 13.22.

Reducing the technogenic impact of zinc production on the environment is of fundamental importance for ChETsZ OJSC, since Russia's largest zinc producer, located inside the city of Chelyabinsk. The sanitary protection zone established by the city authorities does not exceed 1 km.

In view of the above, the company is constantly working on the introduction of environmentally friendly technologies, reducing emissions into the atmosphere, discharges into the reservoir and disposal of solid waste generated in the production cycle.

Despite the increase in production by 10-15%, emissions of harmful substances into the atmosphere have not practically grown in recent years, and the discharge of technogenic products has even decreased by 10%.

The main sources of technological waste gases are kilns, Waelz kilns, and drying drums.

All roasting gases from the KC furnaces are sent for disposal to the sulfuric acid shop. At the same time, they are cleaned of dust, selenium, mercury and other impurities in electrostatic precipitators, washing towers. After extraction from the purified gases in the sulfuric acid shop of sulfurous anhydride, they are released into the atmosphere.

The introduction of a set of measures, the main of which is the double contact system, made it possible to ensure acceptable MPE, and the content of sulfur dioxide does not exceed 0.05%.

The problem of emissions of sulfuric acid mist, the so-called tails, was solved by installing EVMTR - 1 - 2.7 - 36 electrostatic precipitators after the multi-hydrate absorber of the 2nd stage. The implementation was carried out jointly with the Gintsvetmet Institute and the Uraltsvetmetgazoochistka trust. The concentration of sulfuric acid mist decreased by 1.6 times.

At present, the sulfuric acid production is being further modernized, due to which the content of sulfuric acid fog is reduced by 6-9 times, and the unit capacity of the system is increased by 1.8 times.

The problem with the disposal of mercury-selenium sludge has been solved. The introduction of a new method of sludge treatment made it possible to extract mercury from it in the conditions of existing mercury production facilities.

In zinc production, the main source of air pollution is oxide redistribution. The Welz workshop accounts for up to two-thirds of total volume gross emissions, the main of which are sulfur dioxide, zinc - lead-containing dust. During the processing of waelz oxide, the lead cake drying area is the main source of lead emissions into the atmosphere.

Therefore the decision environmental issues in this redistribution was carried out in two directions:

technology upgrade;

technical re-equipment.

The introduction of fluxing additives for Waelz makes it possible to reduce the content of sulfur dioxide in the waste gases of Waelz furnaces from 1.5-2.5 g/Nm3 to 1-1.2 g/Nm3.

A search was carried out for new filter materials to capture dust from Waelz gases. The use of filter bags made of Nomex fabric by BWF makes it possible to increase the degree of trapping by an additional 30%.

Modernization of the gas cleaning equipment of the Waelz shop with the installation of bag filters with pulsed regeneration of filter cloth will reduce the dust content of exhaust gases to MPC (5 mg/Nm3).

Another possible source of dust emissions in this process is the process of drying cakes in drying drums.

A search carried out by factory specialists followed by testing of more advanced equipment showed that it is possible to exclude drying and obtain cakes with the required moisture content (less than 20%) by introducing vertical filter presses from Larox (Finland). The design of these filters allows not only deep dehydration, but also washing of cakes.

The latter is important for the Welz redistribution, since allows further reduction of sulfur dioxide emissions by 60-70%.

At ChETsZ OJSC, water is used to condense water vapor during electrolyte cooling in a vacuum evaporation plant, to cool vacuum pumps in the filter-drying department, to cool clinker in Waelz kilns, to wash Waelz oxides from chlorine, to the evaporative cooling system of the roasting plant and for other purposes.

As mentioned above, despite the increase in zinc production, the volume of water consumption and its discharge decreased by 10%. Discharge reduction Wastewater is associated both with a steady increase in technological discipline, and, first of all, with the development of a number of technical innovations at the enterprise. Organization of plant-wide recycling water supply, as well as local in the main workshops of the enterprise, made it possible to ensure the degree of water circulation equal to 95.6%.

The use of condensate from heating the solutions of the leaching shop for washing off Waelz oxide has been introduced, which made it possible to use both the condensate itself and its heat, and also increases the degree of water circulation.

The use of recycled water has also been introduced at the enterprise, conditionally it can be called the first category (cooling of vacuum pumps, compressors, etc.), for feeding the recycled system of category II (slaking and hydrotransport of clinker, wet gas cleaning, etc.)

With repeated use of recycled water, impurities (especially zinc sulfate) accumulate in it. In addition, when washing Waelz oxide from chlorine and regenerating filters at a water softening station, difficult-to-utilize effluents containing sodium chlorides are formed.

Therefore, it is necessary to treat part of the recycled water and wastewater by reducing impurities to an acceptable level.

Cleaning of the main is carried out at the treatment facilities of the plant, using slaked lime as the main reagent. The resulting sludge containing zinc, cadmium and others heavy metals, which are utilized in the pyrometallurgical divisions of the enterprise. AOOT ChETsZ has developed and implemented a technology that makes it possible to utilize domestic wastewater (volume of about 1000 m3/day) at its own treatment facilities.

During technological process processing of raw materials at the enterprise, the following solid wastes and middlings of production are formed: lead and copper cakes; copper clinker; sludge from treatment facilities and sulfuric acid production (mercury-selenium), as well as used contact mass (vanadium catalyst) and used filter cloth.

Since 1996, the plant has a regulation on the storage and disposal of waste. Sludge disposal has been discussed above.

Copper cakes and copper clinker are completely utilized at copper smelters in the Chelyabinsk Region. At the lead enterprises of Kazakhstan there is a demand for lead cakes from ChETsZ OJSC. However, taking into account that there is practically no production of lead in Russia, the specialists of the plant, together with the Gintsvetmet Institute, have developed a waste-free technology for their processing.

The main object of the technical re-equipment of the enterprise is the zinc electrolysis complex (CEC), the construction of which is carried out according to the project of the Snamprogetti company (Italy).

The commissioning of the CEC has a significant impact on the environmental performance of the enterprise:

due to the transfer of vacuum - evaporative cooling of the electrolyte to cooling in cooling towers, water consumption is reduced by 750 m3 / day, and gross emissions by 110 tons / year;

the commissioning of a new zinc electrolysis and remelting section with installations for the preparation of zinc dust and the processing of drosses reduces dust emissions by 81 t/g (including zinc sulfate - 30 t/g, zinc oxide - 11 t/g), sulfuric acid - 7 t/g , ammonia - 12 t/g, hydrogen chloride - 4 t/g.

hydrometallurgical zinc roasting dust and gas trapping

The ruins of the Electrozinc electrolysis shop after the fire. Photo: Farniev Konstantin

It is no secret that emissions from factories and industrial enterprises are one of the main sources of air pollution, and wastewater discharges from them cause significant harm to the hydrosphere. However, when the work of one plant endangers the life and health of 300,000 people, it is dangerous to remain indifferent.

Electrozinc is the largest industrial enterprise in the Republic of North Ossetia, occupying 70.2 hectares in the north-eastern part of the capital of the region, Vladikavkaz, and one of the leading non-ferrous metallurgy enterprises in Russia, which since 1904 has been producing and selling zinc, cadmium and sulfuric acid.

In addition, Electrozinc is an air pollutant whose activity has caused acute dissatisfaction among Vladikavkaz residents for at least 9 years. Local residents still remember October 5, 2009, when a thick veil formed over Vladikavkaz as a result of an industrial release of sulfur oxide at the plant. The townspeople began to complain of severe headaches and sore throats. On the day of the accident, more than 500 people gathered for a spontaneous rally demanding a referendum on stopping the operation of the enterprise. Until the end of the month, five more emergency above-limit emissions occurred at the plant. Since then, rallies for the closure of Electrozinc have been held regularly. Although the first protests against the work of the enterprise began in 2003: soon after the Ural Mining and Metallurgical Company (UMMC) became its owner, local residents began to complain about harmful emissions into the atmosphere.

It should be added that the sanitary protection zone around the plant is only 300 meters - it is at this distance from it that residential development begins (by law, enterprises of the first hazard class, which is Electrozinc, must be removed from residential areas by at least 1000 meters) . Although the "zone of influence" of the enterprise is far from being limited to one kilometer, but extends to almost the entire Vladikavkaz.

A serious problem is also the places of storage of industrial waste: huge mounds located within the city are open to winds and rains.

The head of the Ministry of Natural Resources of Russia, Dmitry Kobylkin, announced on November 7 that the Electrozinc plant complied with all environmental requirements. “On our line of Rosprirodnadzor, there were never any comments to him. Everything that we asked and demanded, he always fulfilled,” the minister specified.

This confirms the fears that it will not be possible to close the plant and rehabilitate the territories adjacent to it, or at least reduce its emissions to acceptable levels in the near future. Scientist and public figure Tamerlan Kambolov created a petition on the Change.org website addressed to the President Russian Federation with a request to close Electrozinc immediately and permanently. So far, the petition has been signed by more than 26,000 people.

Thousands of invisible sensors installed at the industrial enterprises of the region will help clear the sky over Chelyabinsk, disperse the smog and rein in the polluting plants. Rosprirodnadzor and the Ministry of Ecology have found a way to influence the critical situation with emissions. New project monitoring of the atmosphere will allow you to receive data on the state of the air online around the clock. Indicators from ultra-sensitive gas analyzers will be collected on a single information server and broadcast via the Internet to general access. Such control will allow decomposing the Chelyabinsk air into ingredients and identify those who have gone too far with emissions. To assess the readiness of industrial enterprises to join the “green” project, the Rosprirodnadzor management commission headed by the head of the department for the Ural Federal District Boris Leontiev and the head of the regional administration Vitaly Kuryatnikov visited the Chelyabinsk zinc plant and assessed the environmental policy of the enterprise, where the growth in production volumes occurs simultaneously with the modernization and introduction of clean technologies.

The question hung in the air

In fact, the Chelyabinsk Zinc Plant is located in the city - between the Metallurgical District and the northwest - within the boundaries of a large industrial center. Therefore, the industrial facility has always given priority to the modernization of production and monitored the reduction of the negative impact on the environment. Back in 2011, the plant was one of the first enterprises in Chelyabinsk to sign a contract with the city authorities on cooperation in the field of environmental protection. And today, under this agreement, the results of measurements are transmitted in real time to GorEcoCentre and posted on the company's website. All data is in open access both for supervisory agencies and for ordinary citizens.

To control emissions of sulfur dioxide into the atmosphere during the production of sulfuric acid at CZP, gas analysis systems manufactured by CJSC ENAL, Moscow, are used. The cost of purchasing gas analyzers amounted to 3.6 million rubles, the annual maintenance of the complex is about 350 thousand rubles.

During a visit to the Chelyabinsk zinc plant, the head regional government Rosprirodnadzor Vitaly Kuryatnikov as a member of the commission positively assessed the management's efforts to reduce the negative impact of industrial production on the city's ecology:

— The main objective of the visit is the implementation of the air quality monitoring project. To date, the zinc plant has confirmed its commitment to move in this direction,” said Vitaly Kuryatnikov, head of Rosprirodnadzor for the Chelyabinsk region.

Applicable modern technologies reduction of emissions and environmental protection measures impressed all participants of the commission. As part of the delegation, the enterprise was also visited by the heads of the department of Rosprirodnadzor of the Kurgan region, the Yamalo-Nenets and Khanty-Mansiysk autonomous regions.

— The foundations of the environmental monitoring system were laid in 2011. The zinc plant was one of the first in the city to sign an agreement on cooperation with the city administration in the field of environmental protection. So, for the seventh year now, the results of measurements have been transmitted in real time to GorEcoCentre and to the official website of the enterprise. The same data is sent to Rosprirodnadzor and the administration of Chelyabinsk. Over the past three years, emissions of pollutants into the atmospheric air have not exceeded 30% of the established standard,” said CEO CZZ Pavel Izbrekht.

CZP is currently Russia's largest producer of zinc and zinc-based alloys and the only zinc producer. On the London Stock Exchange, this metal is sold under own brand Chelyabinsk Zinc Plant Special High Grade. In addition to zinc, the enterprise also produces sulfuric acid, indium, and cadmium.

Let's be honest

Over the past nine years, the company has invested more than four billion rubles in environmental protection measures. These funds were used to implement at least six projects for the protection of atmospheric air, about which the CZP management speaks directly:

• acquisition of a mobile atmospheric control laboratory;
• reconstruction of sulfuric acid shop systems and dismantling of technically obsolete shops;
• equipping sources of sulfur dioxide emissions in the sulfuric acid shop with continuous monitoring gas analyzers;
• construction of gas purification systems for kilns of the roasting shop during their start-up and shutdown;
• construction of a bag filter for air purification when receiving pulverized raw materials;
• start of construction of a node for receiving, unloading and granulation of powdered VCS.

The main substance that can have a negative impact on the surrounding air is sulfur dioxide produced during the roasting of raw materials. The waste gas utilization technology implemented in the sulfuric acid shop was recognized as the optimal available technology by ITS 13-2016 "Production of lead, zinc and cadmium". Today, the sulfur dioxide utilization rate at the zinc plant is 99.95%. As a result of the technical re-equipment and launch of the Waelz furnace with the organization of a new dust cleaning room, the dust captured in the filter is returned to production. Implementation this project improves industrial and sanitary conditions in the working area of ​​the workshop, minimizes emissions of pollutants into the atmosphere. Installation costs amounted to 4.5 million rubles.

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Since 2010, CZP's mobile environmental monitoring laboratory has been operating for operational control over the state of atmospheric air at the border of the sanitary protection zone. To create an effective emission control system on the territory of Chelyabinsk, the plant transmits the results of measurements during the NMU periods to email MUP "GorEcoCentre" The cost of purchasing the laboratory amounted to 1.3 million rubles. Air samples are taken at seven points, two of them in a residential area - in the area of ​​Aralskaya and Volkhovskaya streets. The station can catch 15 ingredients in the air: in addition to sulfur dioxide, these are nitrogen dioxide, carbon monoxide, lead, zinc, cadmium and other substances.

According to the comprehensive report on the state of the environment of the Chelyabinsk Region in 2016, published on the website of the Ministry of Ecology for the Chelyabinsk Region in 2016, CZP's emissions amounted to 0.4% of the emissions in the Chelyabinsk Region, or 1.7% of the emissions of the city of Chelyabinsk. Chelyabinsk.

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– Open environmental policy has been carried out at the enterprise since 2009,” sums up Pavel Izbrekht, General Director of PJSC CZP. — During this time, sulfuric acid systems have been replaced and reconstructed, which make it possible to utilize sulfur dioxide formed during the concentrate roasting process. In addition, in 2016, the newest Waelz furnace was put into operation. This is the largest unit worth one and a half billion rubles, which allows processing not only the waste that is generated at our enterprise, but also the waste of other enterprises. We have launched a project for the construction of local treatment facilities, we plan to complete it in 2020. The implementation of this project will completely eliminate the discharge of industrial waste into the Miass River. The implementation of the above measures allows you to gradually reduce the negative impact of production on the environment.

“The more enterprises that will be involved in the projects of Rosprirodnadzor and the Ministry of Telecom and Mass Communications on the territory of Chelyabinsk and Magnitogorsk and, in principle, the region, the easier and clearer it will be for us to work with violators,” Vitaly Kuryatnikov, head of the department of Rosprirodnadzor for the Chelyabinsk region, shares his impression of visiting the shops .

The next step of Rosprirodnadzor will be to verify the reliability of the sensor readings submitted to the supervisory authorities. The information will be made public in order to dispel the veil that hangs over the information space and remove distrust in relation to the data provided on paper. As a result, when the idea of ​​the department is fully implemented, the supervisory authorities and ordinary citizens will be able to see the real picture of air pollution from a particular enterprise, and it will become easier to stop the actions of violators.