Intro

Iron and steel industry is one of the most significant consumers of water. Iron and steel works take the second place after heat generating enterprises and take up to 20% of total water consumption among all industries. 

The complex of steelmaking enterprises includes: 

✔️ enterprises for extraction and processing of ore and non-metallic minerals, which are raw materials for production of iron and steel; 

✔️ iron-and-steel works ; 

✔️ rolling shops; 

✔️ coke plants with coal preparation plants; 

✔️ refractory plant.

All of them can be spread over a large area and, as a rule, have independent industrial water supply systems. There are also large auxiliary facilities that are not directly related to the products manufactured by the enterprise. These are power facilities – TPPs and CHP plants, turbo blower stations, oxygen plants etc. Water supply of these facilities takes 40-50% of the total water consumption.

Water consumers of steelmaking enterprises

Integrated iron-and-steel works have complex water management which contains more than 30 circulating systems, equipped with powerful pump stations, low-water regulation storages, water treatment and cooling facilities. This water complex provides for an uninterrupted supply of needed volumes of water with required quality to consumers and also ensures complete purification of industrial wastewater for its reuse in production. 

Water consumption at integrated iron-and-steel works makes 240–300 m3 per 1 ton of hot metal. Requirements for water quality are defined by production technology. The limiting indicators are: water temperature, suspended solids content, their dispersion, carbonate hardness, salinity. 

Despite the variety of recycling water supply systems water consumers can be divided into five groups, based on the quality of water, used at steelmaking enterprises:

1️⃣ Consumers which use clean water with a content of up to 50 mg/l of suspensions, 1…3 mg/l of carbonate-hardness salts and with a temperature of 28-32°C.

These are coke and blast furnaces, heating furnaces for rolling production, open-hearth and electric smelting furnaces, converters, thermal power plant condensers, steam-air blowing stations, equipment for compressor and oxygen stations;

2️⃣ Consumers which use water with carbonate hardness of 0,5-1 mg/l.

These are continuous-casting plants (cooling of steel moulds) and direct metal shower cooling (sprayers);

3️⃣ Consumers which use water with suspended matter content of  200-300 mg/l. other requirements are similar to those specified in clause 1. 

These installations are: gas cleaning plants for blast furnace, steel and sinter production;

4️⃣ Consumers that do not have requirements for temperature and salt content and have low requirements to suspended substances.

This water is used for industrial waste (scale, ash and slag) transportation;

5️⃣ Consumers that are not directly involved in metallurgical production but have high requirements to water quality. It is water for ventilation systems, steam preparation and domestic and drinking water supply systems.

Wastewater Treatment for Iron and Steel Companies

Metallurgical plants that consume huge amounts of water put the environment at risk of contaminating by wastewater. 

Efficient wastewater treatment and emission reducing measures are currently the  most important issue for the sector.

The water supply system of a steelmaking enterprise includes: 

  • water intake facilities, 
  • water conduits, 
  • distribution networks to water make-up in circulation and fire-fighting water supply systems, 
  • sludge collectors and tailings ponds, 
  • cooling and treatment facilities, 
  • pump stations, 
  • rainfall treatment facilities.

The choice of water supply system depends on:

  • water consumption, 
  • availability and capacity of water supply sources, their distance from the plant site,
  • difference in geodesic water level marks at the water source and factory site.

In modern conditions, when environmental requirements and rational use of water have become crucial, circulating, series-circuit and closed water supply systems for metallurgical enterprises are used.

Circulating water supply systems for metallurgical enteprises

Circulating water supply systems for metallurgical enterprises are the only possible option when water sources capacity is insufficient and when treated water contains toxic substances, purification of which to maximum permissible concentrations in ponds is difficult.

Series-circuit water supply system

A series-circuit scheme of water use is appropriate for short distances between enterprises that use and discharge wastewater. In this case wastewater can be cooled and purified or transferred to the next consumer without treatment.

Closed-loop water supply system 

The use of closed water supply schemes for ferrous metallurgy enterprises is an optimal solution for water management issues, since this eliminates pollution of water sources and provides for almost complete use of useful waste obtained from wastewater during production. 

Zero-discharge water system

Zero-discharge schemes make it possible to use in industrial water supply all wastewater discharged from the territory of the enterprise, reducing to a minimum fresh water intake from water supply sources.

The largest consumers of water and therefore sources of a significant amount of contaminated wastewater (up to 90% of the total amount) at metallurgical enterprises are gas cleaning plants of various metallurgical areas as well as hot sheet rolling mills. Wastewater purification at metallurgical plants is solved independently for individual production facilities.

The industrial water supply system of a steelmaking enterprise has several circulating systems for each shop, which differ from each other in water quality and required pressure in the network. The amount of fresh water ranges from 5 to 8%. The main shops must need continuous water supply. That means that it is necessary to duplicate each line of the network with supply of full calculated water flows. Up to 75% of water is used for cooling structural elements of blast furnaces, steelmaking, heating furnaces, steam condensation etc. This water is only heated up. Water for cooling equipment and products, as well for transportation of mechanical impurities  is heated up and polluted and its amount reaches 22%.

Wastewater Treatment Solutions for Iron and Steel Production

Steelmaking shops usually have two circuits of circulating water supply. In one of them water acts as a heat carrier (cooler) without coming into contact with the product and without receiving contamination; in  the other circuit water acts as a cooler and a medium that absorbs and transports mechanical impurities (metal dust, scale and some others) to water treatment facilities. In the first circuit a ricirculated  water cooler is installed, and in the second, a water treatment plant is installed.

Wastewater from converter gas cleaning plants.

When smelting steel in converters the liquid cast iron poured into them is blown from above with oxygen-enriched air. So 1200-1600 m3 of waste gas per 1 ton of melted steel is produced, containing up to 80 g/m3 of fine, mainly metal dust (98% of iron oxides  and 2% of silicon oxides and manganese). Before being released into the atmosphere this gas is purified with water until its dust content is 50 mg/m3. 

There are several schemes of converter gas purification, the amount of wastewater from which is about 4 m3 per 1000 m3 of gas, or 4.5-6.5 m3 per 1 ton of steel produced. Wastewater from converter gas purification is contaminated with suspended substances, containing about 5000 mg/l from one gas cleaning plant, when the converter is operating for steel production and 8000 mg/l when operating for producing semi-product; the average content of suspended solids in the effluents from two parallel operating gas cleaning plants by production of steel is reduced to 2500 mg/l and from three – to 1700 mg/l due to a more uniform flow of gas. 

Concentration of suspended substances in wastewater during a steel heat in a converter is not constant: at the beginning of the heat it quickly (within 1.5-2 minutes) reaches a maximum of 5000 mg/l or more. When several converters operate in a shop in parallel, concentration of suspended substances is averaged both in wastewater and in water clarified in the settling tank. Suspended substances contained in water are abrasive, despite their very small size. 

Water for the converter gas cleaning plant is recycling, with wastewater clarification in radial or horizontal settling tanks and cooling in cooling towers. In this case wastewater discharge into the pond is not allowed. Purified and cooled water is returned to the gas cleaning plant and for cooling oxygen lances.

To purify wastewater from the converter gas cleaning plant only radial settling tanks are currently used, similar to recycling water supply circuits of the blast furnace gas cleaning plant, which work well with a hydraulic load of about 0.7 m3/h m2. 

The water is purified from the concentration of suspended substances on average 125 mg/l. The resulting sediment in the settling tank is used in the charge of sinter factories and amounts to about 8000 tons of iron per year for one 100-130 t converter. 

In recycling water supply projects of converter gas cleaning plants water treatment with a hydraulic load of up to 1.5 m3/h m2 or more is carried out before settling tanks with a solution of flocculants and coagulants. These flocculants and coagulants are selected in the laboratory specifically for each enterprise. Treatment of wastewater with reagents accelerates sedimentation of relatively large particles by 1.5-3 times, but at the same time slows down precipitation of small particles. 

Sludge from the settling tanks of the converter gas cleaning plant, as well as that of the blast furnace gas cleaning plant should be periodically pumped out with its thickening to a solid to liquid ratio (s:l) of 1:3, which corresponds to a suspended matter concentration in the pulp of up to 300 g/l. 

After that the resulting sludge is pumped into a sludge storage tank or subjected to dewatering on filter presses or vacuum filters. 

Clarified water from radial settling tanks is fed to mechanical-draft cooling towers for cooling and then again to the gas cleaning plant. Thus the cycle is closed.

Wastewater from electric furnace gas cleaning plants.

By production of steel in intermittently operating electric furnaces waste gases are formed, which mainly contain fine metal dust. Before being released into the atmosphere these gases are purified by means of water until dust content in them is 50 mg/m3. 

The gas purification scheme of electric furnaces is similar to that of the converter gas. Water consumption for gas cleaning and the amount of wastewater from the gas cleaning plant is 4 m3 per 1000 m3 of gas or 4.5-6.5 m3 per 1 ton of steel produced. The concentration of suspended substances in wastewater from electric furnace gas cleaning plant is 200 – 500 mg/l or more, and when smelted steel is blown through with oxygen, the concentration of suspended matter in wastewater increases to 2000 mg/l or more. The water supply system of a gas cleaning plant is a recirculating system with wastewater settling tanks.

Wastewater sediment from an electric furnace gas cleaning plant consists contains up to 70% of iron. The density of the dry sediment is about 4.5 t/m3.

Wastewater from washing heat-recovery boilers.

To recover waste gase heat from steelmaking and heating (rolling-shop) furnaces, heat-recovery boilers are used.

Furnace gases, passing through the heat-recovery boiler, give their heat to water, turning it into steam, which is used for industrial or heating purposes. The contaminants, contained in the gases, are largely deposited on the heat exchange surface of the boiler and are removed from it at certain intervals by washing this surface with water. 

Currently, heat-recovery boilers are installed behind almost all open-hearth furnaces, many heating furnaces and converters.

Wastewater from washing heat-recovery boilers is contaminated with mechanical and chemical impurities. Mechanical insoluble impurities are charge carryover products containing up to 84% iron oxides (in equivalent to Fe2O3). 

Chemical contaminants are soluble charge carryover products. 

The discharge of such effluents into a pond is completely unacceptable. At the same time, this water can be used again for washing heat-recovery boilers after neutralizing it with lime and clarification. Therefore the water supply system for washing heat-recovery boilers should include water circulation, neutralization and clarification in settling tanks.

In addition to wastewater from washing heat-recovery boilers there is also boiler blowdown water. The boilers are blown down once per shift for 5 minutes with the amount of wastewater being 0.5-1 m3. Wastewater after boiler blowdowns contains hardness salts. Blowdown water is mixed with boiler washing wastewater or sent to hydraulic ash handling.

Wastewater resulting from cooling and hydraulic cleaning of moulds after steel casting.

The smelted steel is either poured into moulds, installed in the same shop, or sent to a continuous caster. 

After emptying the casting moulds and before their reuse they are cooled with water in a special spraying installation near the steelmaking shop. Then the moulds are subjected to hydraulic cleaning from scale, slag (carbon deposits), lime and varnish (resinous mass). 

The spraying cooler of moulds is switched on from time to time, depending on the productivity of the steelmaking shop. Its total operating time is 4 – 12 h/day, but water is consumed over a longer period of time or even around the clock. The wastewater amount from the spraying cooler is 180 – 360 m3/h, which is equal to approximately 60% of the amount of water, supplied during the cooling period of the mould, the rest of water (up to 40% of its total consumption) is evaporated due to contact with hot moulds and metal. 

The installation for hydraulic cleaning of moulds is also operated periodically, although for a longer time than spraying. Here water is supplied under high pressure, created by a special pump, and flows out in streams through nozzles. The amount of wastewater from hydraulic cleaning of moulds is 25-50 m*/h, which is also equal to 60% of the amount of supplied water; the rest of the water is evaporated.

Wastewater from shower cooler installation and hydraulic cleaning of the moulds contains scale, slag, lime and a resinous mass consicting of varnish and carbon deposits. Total concentration of suspended solids in water is 750-2000 mg/l. Discharge of such wastewater into a pond is completely unacceptable. 

The water supply system is arranged only with water circulation. In this case, up to 40% of the water is lost by evaporation, the rest is purified from contaminants (scale, carbon deposits , lime and coal-tar) in a rectangular settling tank. The total concentration of suspended solids in wastewater is 750-2000 mg/l. In purified circulating water, supplied to showering devices and hydraulic cleaning of moulds, the suspended matter content is allowed to be no more than 100-150 mg/l; sometimes it is required that the suspended matter content in water for hydraulic cleaning of moulds does not exceed 50-75 mg/l. 

To clean wastewater from an installation for cooling and hydraulic cleaning of moulds it is recommended to use settling tanks, as well as to clean wastewater from hot metal casting machines. The time of water being in the settling tank is supposed to be 30 – 60 minutes. Purified water is supplied again for the same purposes without cooling. At the same time approximately 40% of fresh water is added, which is equal to losses through evaporation of water during its use. For obtaining water with a suspended content of 50-75 mg/l the settled water is additionally purified at some enterprises by filtering through a 1 m layer of medium-sized quartz sand (grane size 1 mm). The average water filtration rate is 35 m/h. Pressure filters are backwashed with recycling purified water automatically.

At some enterprises wastewater from the cooling system and hydraulic cleaning of moulds behind steel casters is combined with wastewater from rolling shops. Water for cooling and hydraulic treating of moulds is coming from the recycling water circuit of rolling shops.

Wastewater from continuous-casting machine

Steel, smelted in electric and open-hearth furnaces or converters, is poured into a steel ladle, which is transported by crane to the continuous casting plant. 

Wastewater from continuous-casting machine can be divided into two types: 

– unpolluted water from the mold, pressure rollers, withdrawal-roll sets, gas cutters and leding shoes. The temperature of such water is 5 degrees higher than the incoming water; 

– polluted water from the secondary metal cooling zone. This water is contaminated with metal shavings, scale and oil and contains grains of metal, formed when cutting ingots; the temperature of this water is 5-7 degrees higher than that of the incoming water.

A significant portion of water in the secondary cooling zone evaporates. The rest of the water and vapor condensate from the secondary metal cooling zone is drained into a receiving chute and then into a tank, which is also the primary scale sedimentation tank. From this restank water is pumped into a secondary settling tank or sludge storage tank. 

Water consumption and the equal amount of wastewater per a caster strand are approximately: 300 m3/h of unpolluted water from cooling and 20 m3/h of polluted water. 

Contaminated wastewater from a continuous steel casting machine contains up to 5-7 g/l of suspended substances, consisting mainly (93%) iron oxide (Fe2O3) and 30 mg/l of oil. Suspended particles quickly precipitate from the water in a settling tank. 

Water supply of continuous steel casting plants is arranged as circulating water in two independent circuits (clean and contaminated water) with a contaminated water settling tank or, if it is appropriate under local conditions, the water from the CCM is combined with circulating water of the rolling shop.

There are no standard designs of water treatment plants for continuous steel casters. They are built according to individual projects depending on productivity and water consumption. Contaminated water first enters the primary settling tank (scale pit) in the shop. Then it is pumped with a suspended content of about 300 mg/l into secondary settling tanks.

In order to remove scale and oil from water, standard rectangular settling tanks are used, similar to those for secondary treatment of rolling mill wastewater. The hydraulic size of suspended matter in wastewater from a CCM is the same as that of wastewater from large-section and billet rolling mills. The hydraulic load on the settling tanks is 1.0-1.2 m3/h·m2. 

Mold cooling water is cooled separately in mechanical-draft cooling towers.

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Our Experience in Water Treatment Systems for Steelmaking Plants

Stabilization treatment of the “dirty” cycle water from a BOF shop. At one of the plants there was a problem of high content of suspended solids and hardness salts in the supplied water in the existing circulating water supply system for BOF shop gas cleaning plant with a capacity of 830 m3/h.  This problem led to clogging nozzles and rapid scale formation on the walls of devices, in particular at the adjustable gap of the Venturi tube. The Customer’s requirement was to solve the problem without changing the existing circulating water supply system.

Our team showed a comprehensive approach to solving the issue, which included:

– water sampling at certain points (in trays at the gas cleaning plant exit, in the intake chamber, clarified water after settling tanks); 

– chemical analysis of water and selection of the type and quantity of reagents (coagulants and inhibitors); 

– for scale growth monitoring on the walls of the device we developed an indicator plate to be installed in a Venturi tube. This solution allows you to see the result without stopping equipment; 

– commissioning of reagent treatment of the circulating water supply system; 

As a result of the work performed the amount of suspended substances in chemically treated water, supplied to the gas cleaning system, was less than 95 mg/dm3. 

Normalization of the circulation system work under intensive conditions of the BOF shop operation.

The following stages were completed:

  • Inspection of the gas cleaning plant circulation systems was carried out;
  • The water-chemistry balance of turn-over water from the BOF shop gas cleaning plants was studied;
  • A list of repair and recovery activities was compiled for the period of converter No. 1 re-lining works;
  • Indicator plates were installed;
  • Actual operating parameters of gas exhaust ducts of converters No. 1,2 were determined;
  • Operation parameters of the steel smelting and gas removal were studied;
  • The boiler-cooler OKG-400 of converter No. 1 in the oxygen-converter plant was inspected;
  • Water-chemistry conditions of the boiler-cooler was inspected.

As a result of the work the following recommendations were submitted:

  1. A positive Langelier index indicates possible tendency of water to form carbonate deposits in gas cleaning plant nozzles, on the walls of the gas cleaning devices, in clarified water supply and in slurry pipelines.
  2. To prevent carbonate deposits in the Venturi tube with the chemical water composition being present at the time of research, stabilization treatment of water with an inhibitor was recommended. The inhibitor should be introduced into the water supply pipeline to the Venturi tube at a distance of at least 30 diameters from it. The reagent should be injected only during purging. The maximum inhibitor quantity is up to 20 mg per 1 m3 of water supplied to the Venturi tube. 
  3. To achieve a concentration of suspended substances in clarified water not more than 150 mg/dm3 (under conditions of stabilization water treatment) it is recommended to treat water with a flocculant. The flocculant should be introduced into the slurry pipelines, transporting water from the gas cleaning plant to the classification unit. As part of the installation for reagent water treatment it is also necessary to provide tanks and pumps for supplying a mineral coagulant in the event if suspended particles accumulate in the system.
  4. Deaerators DSA-200 No. 1,2 should be put in operation after repair in order to get deaerated chemically treated water and carry out additional deaeration before supplying water to the steam converters and evaporators .
  5.  Condensate supply from the boiler room to the deaerators DSA-150 No. 3,4. should be provided for.
  1. An evaporator of the power unit should be put in operation and generated condensate must be supplied to deaerators DSA-150 No. 3,4. 
  2. The share of condensate in the feed water should be brought to 90-95%, while water losses in the boiler due to fistulas should be excluded. Maximal condensate return is to be organized.
  3. Maintain a continuous blowdown value of no more than 10% on the boiler-cooler (at the same time the salt content of the feed water is no more than 200 mg/dm3).

M Heavy Technology Principles of Water Treatment in Steel Industry

Water quality analysis:

A thorough water quality analysis must be carried out before developing a solution. This includes measuring the content of various contaminators.

Adaptation to local conditions:

Water treatment solutions must be adapted to the local regulation requirements.

Integration of technologies:

Modern water purification systems often include a combination of different technologies, such as filtration, reverse osmosis and chemical treatment. Integration of these technologies allows for more complete and effective cleaning.

Sustainability and energy efficiency:

Water purification solutions must strive for sustainability and energy efficiency. Using existing energy sources, optimizing processes and minimizing waste helps reduce the negative impact on the environment.

Compliance with safety standards:

When designing and implementing water treatment systems safety standards and regulations must be strictly adhered to in order to prevent risks to human health and environment.

Monitoring and maintenance:

Regular monitoring of water quality through integration of process control systems as well as software and maintenance of equipment are an integral part of the successful operation of water treatment systems. Early detection and elimination of problems provides for the long-time operational efficiently of the system. 

Environmental impact:

When developing solutions, it is mandatory to carry out an analysis and calculation of an environmental impact assessment, the aim of which is to emphasize the absence or reduction of the environmental impact after implementation of design solutions.

Economic feasibility:

Development of solutions must consider economic feasibility. It includes estimating the costs of implementing and maintaining a water treatment system.

Following these principles helps create reliable, efficient and safe water treatment systems that can provide long-term access to clean water.

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