The steel plant of today is exposed to an ever-growing

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1 RAW MATERIALS AND IRONMAKING Methods of reducing costs for hot metal desulphurisation Hot metal desulphurisation is a key process step in steelmaking, and one where considerable savings are possible if the process is properly monitored and run. Almamet GmbH works in close conjunction with plant operations and purchasing personnel on a daily and monthly basis to examine operating costs and provide an all-encompassing reagent supply strategy, allowing steel plant personnel to concentrate on allocating internal manpower to other efforts in the organisation. Author: Steven N Biljan Almamet GmbH The steel plant of today is exposed to an ever-growing need to reduce costs in a dynamic and changing global market. Steel plants employ different individuals or groups to monitor and affix an increasing or decreasing value to this cost, and changes or reallocation of manpower within the steel plant can manifest into personnel having to educate themselves on the technology. Detailed analysis of the desulphurisation process can prove to be significant when searching for the proper cost reduction strategy. Sometimes variables do not present themselves until the cost is already incurred and a corrective action must be taken. Thus the longer this correction is postponed, the larger the losses. Some steel plants monitor and detail areas of cost reduction. Almamet GmbH works in close conjunction with operations and purchasing personnel on a daily and monthly basis to examine these costs. As an allencompassing reagent supply strategy, this can prove a worthwhile effort in reducing costs and constantly reassessing performance with minimal effort from the steel producer. This direction in strategy allows the steel plant to concentrate on allocating internal manpower to other efforts in the organisation. This paper will outline some of the methods of reducing costs for hot metal desulphurisation. ` Injection lance and skimmer paddle ` Selection and consumption ` Skim weights and yield loss ` Accuracy of sampling ` Conformance to aim, efficiencies, over shooting and reshots ` Evaluation of injection data and reporting ` Operator training and process knowhow Cost savings may be realised in the following ways: ` Continuously comparing and monitor results to expected results ` Providing facility spares, ie, valves, consumables ` Reducing responsibility of steel plant for desulphurisation, ie, labour, cost analysis ` Reducting cost through accurate reagent consumption ` Increasing desulphurisation plant availability ` Providing redundancy of storage and equipment ` Training, ie, skimming, process, cost savings ` Providing less expensive Mg blends ` Optimising injections and automation ` Increasing hot metal yield through additives ` Evaluating monthly data for cost reductions ` Providing knowhow from practical examples FULL FACILITY AUDIT This is the first step in achieving savings. It identifies areas such as: ` Reagents and reagent performance ` Off-loading and storage ` Injection system and equipment ` Injections (ratios, rates, injection turbulence) ` Evaluation of sulphur sampling practice ` Automation and control (simplifying operations, interlocks, troubleshooting) ` Maintenance, spares and equipment redundancy Reagents and reagent performance In many plants reagent preference has moved away from calcium carbide/mg as a desulphurising reagent towards lime/ Mg. In plants with operating temperatures of ~1500 C and below, lime and magnesium have become the more cost-effective solution. Other factors are the inherent danger of calcium carbide when there is a spill or exposure to moisture, the cost of the reagent and its availability. An example of savings from a plant that changed its reagent is shown in Table 1. Reagent quality has increased substantially as technology has matured. Almamet has pioneered customised reagent a 49

2 r Table 1 Example cost comparisons when using Mg/CaC 2 co-injection and Mg/lime co-injection mixtures for more than 20 years. Mg/lime and Mg blends containing secondary magnesium have been developed to reduce cost/t, while maintaining the effi ciency of magnesium-based desulphurisation reagents. r Fig 1 Plant with storage silos r Fig 2 Operator screen using a setup with horizontal tankers Off-loading reagents and storage The redundancy of reagent storage and plant availability is key to the delivering of reagent into the hot metal. Some steel plants have moved away from the initial capital cost of silos and storage for lime and Mg such as shown in Figure 1 (lefthand side). This is accomplished by agreements between the reagent supplier and the steel plant to supply on-site storage via moveable horizontal storage tankers for lime and Mg. These automated trailers are replaced when empty. Total hook-up time is approximately 15 minutes. Typically, two tankers per reagent are always on site. This decreases potential downtime due to silo issues and dramatically reduces off-loading and consumption of nitrogen gas. Figure 2 shows a typical operator screen using a setup with horizontal tankers. Injection systems and equipment As co-injection systems vary from plant to plant, the optimisation of the injections, performance of the entire system with respect to ease of use, monitoring, proper valves and components, reliability, redundancy and ease of maintenance are key to consistent plant availability. To standardise and improve overall operations, many plants have upgraded existing equipment in the form of entire injection systems, provided additional redundant systems, upgraded certain components of the existing system and upgraded their automation. 50 Injections (ratios, rates, turbulence) When magnesium is introduced into the hot metal, most of the magnesium changes to a gas, causing turbulence and stirring. The effi ciency of magnesium injection increases with lance depth as there is more time for the reaction to occur. A proper balance of vessel pressures for Mg and lime, orifi ce sizing, transport gas fl ow and hot metal back pressure are critical for a proper injection and these factors are customised for ladles that may range in size from 80t to 305t.

3 RAW MATERIALS AND IRONMAKING r Table 2 Example potential cost reduction from reducing ratios Lime to magnesium ratios are typically 3:1, but are dependent on ladle size. On smaller ladles, ratios tend to be 4:1 or above, due to the lower Mg rates required and providing enough carrier reagent (lime) to prevent the lance from plugging while reducing turbulence. On larger ladles, ratios of 2:1 are not uncommon, especially when aiming for low sulphur (10-20ppm) and to reduce the cycle time. Mg rates are typically 6-14kg/min for t ladles and 15-23kg/min for t ladles. Temperature and freeboard play a large role in the variability of these rates and efficiency tends to be lower with higher rates. If time permits, lower Mg injection rates are preferred due to higher efficiency, but this must be weighed against the additional lime that must be delivered during the longer injection time. A plant example of savings by altering ratios is shown in Table 2. Figure 3 illustrates poor control (high turbulence) and good control (low turbulence) over injection rate. r Fig 3 Examples of poor (left) and good (right) turbulence control AUTOMATION AND CONTROL r Fig 4 Typical operator co-injection screen a The aim for automation should be to simplify operations, provide interlocks and assist in troubleshooting, and can be either an excellent advantage or can hinder the operation if not setup properly. A well-designed system allows the operator a detailed window into the process and accurately depicts all the operations, monitors key process and programmed variables, options, solutions and events, both intentional and unintentional. Automation systems must allow for flexible control while still assuring safety during operation, idle situations and especially during troubleshooting. With today s control systems, ease of use and troubleshooting is an important tool on a heat-by-heat basis. Ease of use would involve minimal input from the operator while still allowing for detailed control and options. Minimal input eliminates most human error, from unintentionally starting sequences to entering improper data that would later produce distorted reports. Detailed control of the system must also be available to allow for proper troubleshooting to minimise downtime and delays. Figure 4 shows a typical operator screen. Operator training on the control system is a crucial tool in eliminating downtime and preventing unnecessary 51

4 r Table 3 Desulphurisation plant availability r Table 4 Example cost savings with increased lance life delays. Almamet has extensive experience in automation and control of desulphurisation systems around the world being able to advise and assist on operator training, troubleshooting process and control system issues. 52 r Fig 5 Typical skimming operation Maintenance, spares and equipment redundancy Improving plant availability is a large factor in reducing desulphurisation costs. Maintenance of equipment in a timely manner is critical to proper availability of the entire facility. Maintenance can involve operations, from changing lances, skimmer paddles, replacing valves and instruments, repairing or replacing piping, and repairing Leco sulphur analysers, to more involved operations, such as skimmers and transfer cars. A majority of plants have recurring problems that are outside their knowhow, only because they do not have experience of other plant systems to use as a comparison. Most typical areas of redundancy involve lances or lance hoist assemblies, operator work stations, injection equipment, skimmers and Leco machines. Facilities where two stations exist on the same platform have the luxury of redundant injection systems, skimmers and a common pulpit that can cross-inject to the other station in times of maintenance repair. A double skimmer in this situation also helps, but still consumes valuable time to bring the ladle to the other skim area. Redundancy in material storage was covered earlier. Almamet has assisted many plants in overcoming recurring issues with equipment by modifi cations and upgrades to equipment and in-depth analysis of process problems and their solutions. An example is shown in Table 3, which shows the relative additional cost per heat to desulphurise at a BOF because the desulphurisation facility was not available to treat the ladle.

5 RAW MATERIALS AND IRONMAKING r Table 5 Example of potential cost savings with slag yield loss reduction additives Injection lance and skimmer paddle selection and consumption Lance life is usually measured in minutes or heats on lance, or both, and generally varies from plant to plant. Ideally, a detailed lance tracking report comparing different plants within the same organisation on a monthly basis, would be preferable. The report would outline each individual lance used throughout the corporation, covering minutes, heats, date implemented, date removed and cause of failure with each respective lance supplier. This report would preferably be co-ordinated by a third party with unbiased evaluation of the entire corporation. The data would be assembled into a main report presented to corporate management or purchasing with recommendations for improvement. Lance performance, and thus suppliers, would be evaluated corporate-wide, with detailed data to correct negative performance trends. Several factors would become evident with lance performance. Comparison between suppliers would vary, in some cases substantially, not only within the same plant but also between plants as well. This would produce a rising competitive trend among suppliers to become increasingly targeted to quality and to providing improved refractory formulae to increase minutes on lance. Small ladles with small lances produced very high lance life, in some cases more than 2,000 minutes. The longer lances, especially with dual port functions with aggressive injections, fare much worse, sometimes with only 400 minutes per lance. Suppliers that produced better results for the month would then be the preferred suppliers. Table 4 shows an example of cost savings with increased lance life. Skimmer paddle life is typically not tracked in the same manner as lances so a tracking method for skimmer paddles can be implemented. Skim weights and yield loss Skim weights vary, due to several factors such as blast furnace carryover slag, hot metal temperature, agitation, injection rates, use of nitrogen bubblers and operator skimming practices. Calcium carbide in the co-injection process historically has high skim weights, and iron or yield losses have been difficult to measure. Several methods using a 1t sample or smaller have shown the best results for measuring the amount of Fe in the slag. Overall, slag weight reduction, compared to results without slag treatment, prove to be substantial. Due to desulphurisation slag being stiff and entrenching iron within the slag, slag conditioners are used to create a more fluid slag. Cryolite is used as a slag conditioner in Europe, providing a source of Na and F, creating a very fluid slag with no effect on ladle refractory. Steel plants that have weighing systems on transfer cars or cranes are at an advantage when tracking and monitoring performance of slag conditioners, and in operator practices measuring the hot metal weight before and after treatment and skimming. Improved operator skimming practices, hot metal temperatures and cycle time improvements have a direct effect on skim weights. Figure 5 illustrates slag skimming. Potential cost savings are shown in Table 5. Accuracy of sampling Sulphur analysis is the cornerstone of hot metal desulphurisation technology and Leco sulphur analysers are the preferred method for accurate and reliable results. Steel plants with such analysers within the control pulpit, have an advantage in that they give faster results over remote central laboratories. Typically, analysis that is performed in the pulpit is performed on two separate analysers. One is dedicated to the start sulphurs and one to the final sulphurs, and they are calibrated accordingly for high and low S ranges, respectively. A wide spread of results for similar heats is usually indicative of poor sampling and, although other areas such as inhomogeneous ladle contents may be the cause, this is usually the first and simplest point to examine. When evaluating injection data with these inaccurate results typically, the Mg/t of hot metal for a specific aim is an indicator of this sampling error. Conformance to aim, efficiencies, tolerances, overshooting and re-shot Conformance to aim is measured as a percentage of heats that fall within a tolerance band for a given aim and is an analytical tool for measuring performance of kg/t reagent additions. Target aims such as a 53

6 RAW MATERIALS AND IRONMAKING r Table 6 Example of potential cost savings with reduced Mg/t within target aims r Table 7 Example of potential cost incurred by over-shooting heats /-1ppm or 20 +/-1ppm usually have a conformance of % over all start sulphur ranges. Target aims above 50ppm typically have a lower conformance to aim because of the difficulty in guaranteeing that a specific amount of sulphur will react with the reagent. Thus, these aims have a greater tolerance associated with them. Efficiency of magnesium use is measured as a percentage and is the relationship between the difference of start and final sulphur with tonnes of hot metal treated per Mg units injected. Magnesium efficiency decreases significantly at lower aim sulphurs due to the increased Mg required to account for the low amount of sulphur units remaining. At higher start sulphurs and lower aims this efficiency tends to increase due to the longer injection time. Tolerances when referring to aims, identify the allowable variance that is permitted for a final sulphur reading as compared to the targeted aim. Savings are realised when falling at the outer limits of this range and must be balanced with BOF steel grade. Typically, a two point higher result in final sulphur is allowed on the higher aims and a one point allowance on the lower aims. Example savings are shown in Table 6. Overshooting or over treatment of a heat refers to unnecessary excess Mg or carrier injected to produce a lower final sulphur than was required. This is usually evident in the conformance and must be determined if the conformance was positive or negative with respect to the aim. Other factors that affect overshooting occur when operators are able to inject more than the allowable reagent batch weight to guarantee an aim without reasonable justification. This becomes evident in the detailed analysis, and if left unchecked can incur high costs over time. Table 7 shows an example. Re-shots or re-treatment is identified as the continued treatment on an existing heat. The final sulphur is not within tolerance and must be re-shot. A significant amount of re-shots that suddenly occur represents a loss of process control either in sampling or other factors. Evaluating injection data and month end report With a full facility audit aimed at cost reduction, Almamet would create a report at the end of each month that would outline savings over the previous month on a per plant or corporate-wide basis, examining reagent costs and plant performance. Other savings would be above and beyond raw data and would reflect changes to process, equipment or activities within the desulphurisation facility and would be outlined in the report. All areas examined would be with the goal of reducing costs among all aim distributions. CONCLUSIONS The methods of cost reduction have proven to work on a large scale with interaction and forward-looking goals for improvement between the reagent service supplier and the steel producer. Specific knowhow with a common goal produces cost savings that remain intact through improved practices and detailed analysis. MS Steven N Biljan is a desulphurisation specialist at Almamet GmbH Contact: biljan@xplornet.ca