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Darfo S.R.L., located about 150 km northeast of Milan, manufactures primarily ferrochromium and manganese silicon products used as raw material for special steels and stainless steel production.
Founded in 1893, Darfo has a century of operational experience and was one of the first foundries to experiment with a smelting reduction process in an electric arc furnace.
The company continued its tradition of innovation by developing a process to separate molten metal and slag as they flow out of smelting furnaces. The metal is diverted to a channel and flows into a container, where it solidifies. Channeled to the other side, the molten slag - an unusable by-product separated during the reduction of the metal - was also left to harden in a container but was difficult to handle, since the large irregularly shaped lumps had to be broken up and disposed of, resulting in major handling and disposal problems.
Darfo realized, however, that the slag could be recycled as an ingredient for cement products, if it could be reduced to a finer granular form.
To accomplish this, Darfo developed a process that mixes molten slag with water as the slag emerges from the plant's two 6,000 kW smelting furnaces. A sump was built to replace the slag containers. From the sump, the slag slurry is pumped to a raised tank, where the water is drained for recycling. The dewatered, granular slurry is then shipped by truck to cement plants for reuse in cement products.
The highly abrasive nature of the slag slurry inflicted extreme wear on Darfo's vertical pumps, however, and the linings had to be replaced as frequently as every 20 to 25 days, resulting in high maintenance and operating costs.
When Darfo decided to install a new 10,000 kW furnace for ferrochromium - which would produce an even larger and more abrasive quantity of slag - it contacted ITT Flygt in Italy to seek a more economical and practical pumping solution.
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Submersible slurry pumps for abrasive media
ITT Flygt manufactures a range of submersible pumps specially designed to deal with the problems caused by such abrasive media as slag slurry. The pumps feature a newly developed slurry seal system and can be equipped with alternative abrasion-resistant materials. Several design innovations also provide better and easier access to internal components for the shortest possible downtime when the pumps are serviced, or when parts have to be replaced.
For Darfo's new furnace, one of the existing vertical pumps was replaced with an ITT Flygt 5540 to test how it could pump a mixture of foamed slag containing 2 - 3 percent chromium oxide. The specific gravity of the mixture was about 1.2 kg/dm3. The slag granules varied between 0.1 mm and 3 mm in size.
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The pump was installed in a circular pit with a diameter of 1600 mm, with a high-speed side inlet to rotate the entire volume in the same direction as the impeller, and to keep the slag particles in suspension, creating a mixture that was as homogeneous as possible. The water quantity was increased to maintain the temperature around 70 degrees Celsius. Cold water was also circulated in the motor for better operational reliability.
The arrangement produced very good results and the flow decreased only after 250 running hours. The service life of the ITT Flygt pump proved to be about two and a half months, compared with 20 to 25 days for the existing vertical pumps. The advantages of the ITT Flygt pump for ease of maintenance and service (see below) also cut operating costs sharply.
Current installation
Based on the test results, Darfo decided to purchase four H 5550 pumps (one for each furnace and two standby). Modifications were also made to the pumping system to improve the flow process and increase operating reliability.
Among other changes, it was decided to increase the flow and head of the pumps. The slag is now pumped to two 15-meter-high dewatering silos at a flow of 45 1/s and head of 24 m.
After several tests with various wear ring materials, the H 5550 pumps are now each equipped with a wear ring of hardened steel. The cost of routine repair work to replace the wear parts is far lower, compared with maintenance of the vertical pumps. The mechanical seals have shown no sign of damage.
Today, each smelting furnace produces approximately 80 tons of granulated slag daily. The plant manufactures about 30,000 tons per year of ferrochromium and 12,000 tons of ferro-silicon manganese. The pumps are operating The slag is pumped to filtering silos by an H 5550 pump. four hours per day. Each pumps 650 m' of mixture, of which 107 m3 is water-saturated slag. The average specific gravity of the mixture is about 1.08 kg/dm3.
The pump problem previously experienced by Darfo was solved and has further encouraged the company's engineers to develop a new granulation process to separate the slag perfectly from the metal, removing all impurities.
New seal system
The pumps used by Darfo come from ITT Flygt's abrasion-resistant range, which consists of two series, the H 5500 and H 5600, in which all pumps are equipped with a new seal system to prevent abrasive particles from penetrating the motor unit.
The outer seal is protected since it is separated from the wet end by an isolation zone. With this arrangement, the only pressure on the outer seal comes from the medium in which the pump is submerged.
Other design benefits include an expeller, consisting of a rotating plate attached to the shaft. Positioned at the entrance of the isolation zone, the expeller protects the outer seal unit from direct particle impingement.
The seal rings are made of tungsten carbide, the stiffest material available. A special solid construction has been selected that minimizes thermal movement of the seal surfaces, which can lead to particle entrainment. The spring system holding the two mating rings together in the seal unit is also protected from contact with particles in the pumped liquid.
Monitoring seal function and split-case design
The oil chamber of the pumps is equipped with an emulsion sensor to detect water in the oil and indicate when it is necessary to change the outer seal unit. A miniature float switch has been incorporated for leakage protection in the stator housing.
With a split-case design, the pumps can be easily opened to replace the internal linings. By simply removing a few bolts, the drive unit, with the impeller and wear cover still attached, can be lifted out of the pump house for replacement without dismantling the pump house.
The impellers, pump house linings and other wear components are available in either polyurethane or high chromium alloyed, cast-iron versions. The polyurethane alternative is used for medium particles under 6 mm in diameter, while the hard metal option is intended for media with larger, sharper particles, or environments in which the temperature exceeds 50 degrees Celsius.
The H 5550 pumps at Darfo's foundry also have highly efficient three-vaned impellers. The necks of the impellers can be flushed with clean water and the suction cover is adjustable. All of these features contribute to greater pumping efficiency, reduce wear and cut running costs.
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| Facts |
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| Plant: |
Darfo S.L R |
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| Owner: |
Assiatierie Venete |
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| Flygt Equipment: |
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| Pumps: |
4 x HS 5550, 2 x HS 5540 |
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| Application: |
Pumping of slag from electric arc furnace |
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