Carbon Dioxide Injection System Controls pH to Prevent Scaling

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Carbon Dioxide Injection System Controls pH to Prevent Scaling

The blast furnace (BF) is a critical process unit in the iron and steel industry that converts iron ore into molten iron, and is the beginning of the process in the manufacturing of steel. The basic oxygen furnace (BOF) is a vessel that is used to reduce carbon in liquid iron from approximately 4 percent, down to 0.04 percent. The combustion of carbon in the BOF is enabled by blowing pure oxygen into the melt.

During the operation of both, great quantities of gas are generated, containing very fine particles of dust that must be removed. Scrubbers are used to clean the gas with water, producing wastewater with high pH and elevated temperature. The purpose of wastewater treatment is to allow for recycling of the process.

The challenge
Lime is added to meet the requirements of steel customers in the blast furnace process. A wet scrubber device is used to clean the exhaust gases of combustion of various pollutants and dust particles.

The pH of the scrubber water is very alkaline (pH greater than 12.5). At pH levels greater than 9.0 there is an increased risk of calcium carbonate precipitation in process lines, which would result in serious scaling problems. Steel mills are sometimes forced to shut down a number of BOF units due to scaling problems, both from the BOF/BF scrubber water circuit or the cooling water circuit. Reduction of the scaling problem can be achieved by feeding the system with a softening agent like CO2.

The Praxair proposal
Carbon dioxide gas, written as CO2 (g), will dissolve in water, written as CO2 (aq). The dissolved CO2 (aq) will react with water molecules. It will combine with water to H2CO3 (carbonic acid). This step is called hydration.

The dissolution of CO2 in water leads to the production of H+ and thus, to a decrease in pH. With alkalinity and pH, the total amount of carbon dioxide dissolved in the water can be calculated. Both parameters decide if the water can take up more CO2 or if it is already saturated with CO2. The alkalinity also influences the amount of CO2 to adjust the water to certain pH value. The higher the alkalinity of the water the more CO2 is needed to reach a certain pH.

The results
The BF/BOF scrubber water circuit in steel mills is as follows:

  • Wastewater is produced in the gas cleaning system of the BF or BOF scrubber with high pH and temperature.
  • Thickener or pre-settling tank, where approximately 30 percent of the solids are removed.
  • Subsequently, wastewater flows through clarifiers, where most of the solids are removed.
  • The resultant overflow is collected in a buffer tank from where part is recirculated into the BF/BOF scrubber system and the rest is finally discharged to the sewer.
  • Cooling towers are used in the middle and at the end of the process to reduce the temperature.

Praxair’s CO2-based pH control system can be installed at several injection points in the process:

1) in the thickener

2) at the inlet or/and outlet of the clarifier

3) throughout the cooling water loop

4) before the final discharge of water

The CO2 gas injection system can be controlled manually or with automatic feedback based on the pH of the wastewater or its conductivity. Various target points throughout the process were chosen since pH values differ throughout the process. For example, a higher pH is desired (10 – 11) in the thickener for the precipitation of calcium and magnesium carbonates to reduce scaling potential. Lower pH is required (less than 9) in the discharged wastewater to meet the effluent target.

CO2 can be injected through diffuser grills, sintered metal, pump/venturi and loop contact, depending on the application and the existing installation.

Praxair’s CO2-based pH control system allowed the steel mill to reduce the following:

  • Clogging and scaling of transfer lines
  • Acid handling and storage
  • Downtime for cleaning and dredging
  • Maintenance costs

The system also helps maintain compliance with wastewater regulations for suspended solids, metals, sulfites and pH.

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