Indias largest private-sector iron and steel manufacturer, Tata Steel, not only enhanced the productivity of its iron-ore pelletizing process but also found a way to cut the equipments power consumption. How?
It recently adopted a high-performance polyurea grease to lubricate the wheeled metal grates that carry the iron pellets through an induration furnace, where temperatures reach more than 1,300 degrees C. By doing a better job of sealing the gaps between the sliding grates and their housing, the grease prevents the ingress of unwanted air that causes temperature fluctuations in the furnace and undermines its efficiency. The previous product – an NLGI 2 grade lithium complex grease – had not only failed at this, but it melted and oozed from the gaps, creating messy spills and a fire hazard.
The overall improvement in lubrication also brought the peak power draw on the transport systems main motor down to around 115 amp from a one-time 125 amp, and allowed the facility to lower its grease consumption to just 20 barrels a month – a 28 percent savings.
In a presentation to the NLGI-India Chapters Lubricating Grease Conference in February, the companys P.K. Mahato and Ajit Verma said that Tata Steel commissioned the 6 million metric ton/year iron ore pelletizing plant in 2011 in order to increase hot metal output at its steel plant in Jamshedpur. Up to that time, Jamshedpur had been making roughly 6 million tons of crude steel a year, but it aspired to 10 million. That goal is in reach now, and the company says Jamshedpur delivered 9.7 million tons of steel in the 12 months ending March 31, 2015.
Tata Steel, which was founded in 1907, sources much of its iron ore from captive mines in India, and it wants to extend the life of these reserves as much as possible – down to the finest particles. The pelletizing plant was put up to facilitate the process of converting iron ore fines into uniformly sized iron-ore pellets, Mahato explained at the conference in Chennai, India. The pellets can be charged directly into a blast furnace for highly efficient iron making.
Jamshedpurs pelletizing plant was designed and built by the Finnish company Outotec Oyj, at a contract cost of more than Euro 70 million. It employs a three-step process to form and harden the pellets, beginning with ore fines that are too small (0.045 mm and less) for making into sinter. Such particles once would have been scrapped as waste, but today the raw material is ground, mixed with binders, and moistened. When tumbled in massive balling machines, the mixture agglomerates into spherical lumps (green pellets) that measure 9 mm to 16 mm in size.
Green pellets have low mechanical strength, however, so they next are spread onto traveling grates that slide along a track through the induration furnace. Here the grates and their loads endure updrafts and downdrafts of drying air, and pass through zones of blistering heat generated by burners that line the length of the furnace. After enduring temperatures between 1,300 and 1,350 C, the grates finally hit a cooling stage before rolling out of the furnace with their payloads of hardened pellets. These can go into the blast furnace to make hot metal.
Mahato, who is Tata Steels senior manager for lubricants, said the induration furnace usually runs with a gas pressure slightly below ambient pressure to avoid emissions of gas from the furnace. However, this creates a risk that false air will enter the furnace and make it difficult to sustain the required process temperature. To prevent this, a central lubricating system pumps grease at timed intervals into the gaps between the grate cars and their housings, to block the entrance of air. The grease also facilitates lubrication between the moving grate cars and the fixed housing, and thus avoids damage and reduces power consumption.
This is a once-through lubrication process, with almost 450 grease points strung along the slide track and under the hood. No grease is capable to withstand temperatures of 300 degrees C and more, Mahato reminded the NGLI-India Chapter audience. Therefore, finally the grease will burn off or evaporate leaving only a small part of ashes.
However, there were several problems with the original grease Tata Steel was using, Mahato said. The grease was melting and creating spillage which was a fire hazard. The grease was not able to provide complete sealing since the temperature was very high in the contact area. Higher temperature was also resulting in ineffective lubrication of the slide bar and pellet car, causing excess consumption of power for the indurating machine. The plant also had problems in pumping the NLGI 2 grease, and felt that grease consumption rates were excessive.
Since temperature has a very strong impact on the viscosity of oil, and more than two-thirds of grease typically is base oil, greases cannot provide stable properties over a wide temperature range, Mahato said. They will behave differently at elevated temperatures than at lower ones.
Between 100 degrees C and 200 degrees C, some greases display stronger softening and even become liquid depending on the type of thickener and characteristics of other ingredients, he observed. However, at even higher temperatures most of the thickeners will disintegrate and the grease will become fluid. Only a very few have a tendency to stiffen and become paste-like – and such paste provides excellent sealing of the gap in between the housing and the traveling grate car. Therefore, greases which stiffen at high temperatures are recommended as sealing greases for traveling grate furnaces.
Some plants apply a standard lithium-thickened NLGI 2 grease with extreme-pressure additives to accomplish the sealing function in traveling grate furnaces. But Mahato said such products were completely unable to maintain a satisfactory sealing function at Tata Steel. Up to 180 degrees C, certain stiffness may remain, he said, but at 200 degrees C they will become completely liquid and pour from the gap. This requires a huge amount of grease to be pumped. The fresh grease needs to enter faster than it is becoming liquid, otherwise the gap is open and false air may enter the furnace.
A stiffer grease provides better sealing properties and, therefore, higher process stability since the risk of the entrance of false air is not there, he continued. It also results in lower spare-parts costs as there is less wear on sliding plates of cars and the hood, and less power consumption due to better lubrication and reduced friction in between cars and hood.
Tata Steel tried using an NLGI 2 lithium complex grease made with a 220 viscosity mineral base oil. This gave better sealing properties than standard lithium EP greases, but when heated to 200 C it also softened unacceptably and was not capable of providing sufficient sealing.
To overcome the problem, Tata Steel partnered with Carl Bechem Lubricants. Together they studied technical data sheet of various greases, reviewed the whole application, and identified the parameters that could be affected by a changeover. Compatibility tests of the existing grease and proposed new grease were also done to confirm the new grease could be safely introduced without adverse reactions.
The search found that mineral oil based high-temperature polyurea grease that could remain stiff at 200 C due to its tetraurea thickener system. This grease was tested at a constant 200 C (although the application temperature can range up to 300 C). Also, grease feeding is continuous through the centralized grease system so there are no chances for grease to get stuck up for long time, Mahato pointed out.
Comparing the two greases, one notable difference is that the candidate greases base oil viscosity is more than double that of the prior grease (Figure 1). This was seen as a plus for lubricating properties, but it also meant that the plant had to expect a higher pressure loss of the grease flow inside of the pipes of the lubricating system, Mahato said. To compensate, the polyurea greases NLGI consistency was set a little lower, at NLGI 1.5, in place of NLGI 2. Once it reaches the lubrication point and is heated, though, the grease stiffens enough to provide the best sealing effect.
Before initiating field trials, several key criteria were set to gauge the performance of the new grease. One was the current draw of the indurating machines main motor. Data trending showed the peak motor current dropped by 5 amp within days after the changeover to the polyurea grease.
Tata Steel also watched for changes in specific heat consumption for heating the pellets in the indurating machine, and shifts in off-gas temperature at the suction box. Those results are being monitored internally, and were not shared with the Chennai audience.
The audience did hear of changes in the furnaces grease feed rates (Figure 2). Tata found that it could optimize its lubrication systems on/off cycles significantly, and use much less than before. It takes eight fewer barrels of grease a month to keep the traveling grates sliding on their way now.
With the new grease put into service last August, Tata Steel concluded it had found the answer to its problems. Talking later to LubesnGreases, P.K. Mahato reiterated that Jamshedpur has reduced grease and power consumption and enhanced productivity of the pellet plant indurating equipment. He also emphasized the need for thorough analysis of an application before changing to a new lubricant. A lubricant can be suitable for a particular application but it may so happen that it will not be very much suitable for other applications, he remarked.