Productivity improvements through recovery of pickle liquors with the APU process

Transcription

1 Continuous recovery of pickle liquors saves acid and reduces waste treatment expense as well as providing a means of decreasing pickling times and ellmlnating downtimes for bath replacement and tank clean out. Reductions n energy consumption and fume emissions are also possible. Productivity improvements through recovery of pickle liquors with the APU process Craig J. Brown, Executive Vice President, Eco-Tec Ltd., Pickering, Ont., Canada Reprinted from RON AND STEEL ENGNEER 0 Copyright, Association of ron and Steel Engineers

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3 Continuous recovery of pickle liquors saves acid and reduces waste treatment expense as well as providing a means of decreasing pickling times and ellminating downtimes for bath replacement and tank clean out. Reductions in energy consumptlon and fume emissions are also possible. Productivity improvements through recovery of pickle liquors with the APU process Craig J. Brown, Executive Vice President, Eco-Tec Ltd., Pickering, Ont., Canada PCKLNG is the chemical removal of surface oxides or scale from steel by immersion in an aqueous acid solution. While wide variations are possible in the type, strength and temperature of the acid solutions used, sulfuric and hydrochloric acids are the most common pickling acids for carbon steel. Mixtures of nitric and hydrofluoric acids are generally used for stainless steel. Pickle liquors become contaminated with dissolved metals through use. As the metal concentration increases, the free acid concentration decreases and pickling efficiency drops. Additions of fresh concentrated acid are made from time to time to rejuvenate the bath but eventually it becomes spent and must be discarded. Pickling speed varies continually throughout the life of the bath and it is difficult to avoid either under or over-pickling. While most pickle acids are relatively inexpensive, the indirect costs associated with pickling go well beyond the cost of the acid consumed. Some of these indirect costs include: Labor to make up fresh acid. Labor for removal and disposal of spent acid. Cost of neutralizing chemicals. Ultimate disposal of resulting solid waste. Reduction in productivity resulting from the inhibiting action of dissolved metals. Lost production time that occurs while spent acid is removed and replaced. Quality control problems due to over and under-pickling as bath composition changes. Recovery of spent pickle liquors can potentially reduce many of these costs. Various techniques have been employed to recover waste pickle liquors. These systems are based on a variety of unit operations including evaporation, crystallization, roasting and solvent extraction. Unfortunately, most are expensive and generally not suitable for any but the largest and most technically sophisticated operations. A simple, low cost unit, called the acid purification unit (APU), was introduced in 1978 in North America for recovering spent mineral acids. Since that time, several hundred of these systems have been installed around the world, primarily in metal finishing applications such as sulfuric acid aluminum anodizing, sulfuric/peroxide brass and copper etchants and various aluminum, brass, copper and nickel etchants employing nitric acid.l Recovery of pickle liquors with the APU has recently attracted considerable attention in the steel industry and a large number of units have been installed over the past couple of years for this application. The APU process Operating principle - Certain ion exchange resins have the ability to sorb strong acids from solution, while exclud- ing metallic salts of those acids. The process is reversible, in that the acid can be readily desorbed from the resin with water. t is thus possible, by alternately passing contaminated acid and water through a bed of this resin, to separate the free acid from the metal. Unfortunately, only small volumes of solution can be processed each cycle. The difficult part of the process is finding a way to efficiently elute purified acid from the resin without contaminating it with the impure feed acid and without excessively diluting it. A novel ion exchange technique called Recoflo, which has been extensively used for recovery of metals from metal finishing wastewater,2 has proven ideal for this application. Through the use of short resin beds, fine mesh resins, counterflow regeneration and various other features, the Recoflo technique provides the necessary tool with which to achieve the required separation efficiency. The resulting system is called an acid purification unit or APU. There are two steps in the basic APU process-the upstroke and downstroke (Fig. 1). During the upstroke, contaminated acid is pumped into the bottom of the resin bed. Acid is sorbed by the resin particles and the remaining deacidified metallic salt solution, called the by-product, is collected from the top of the bed. Next, during the downstroke, Flg. 1 - APU operating cycle. UPSTROKE WATER water reservoir water 4 DOWNSTROKE METALLC SALT BYPRODUCT (WASTE) PURFED ACD PRODUCT SPENT ACD m January 1990 ron and Steel Engineer 55

4 water is pumped into the top of the bed, desorbing the purified acid from the resin so that a purified acid product is collected from the bottom of the bed. The total cycle typically takes approximately 5 min to complete and continuously repeats itself. Equipment and layout - The heart of the APU is the resin bed which is typically 30 to 60 cm (12 to 24 in.) in height, depending on the application. Scale-up is accomplished by increasing the diameter of the bed. Units are constructed in a range of capacities. Small units with bed diameters from 15 to 50 cm (6 to 20 in.) utilize hydropneumatic reservoirs to pump the feed acid and elution water through the resin bed. Larger units, up to 180 cm (72 in.), utilize electronic measurement of flows and are equipped with external holding tanks and pumps. A typical unit equipped with a 107-cm (42-in.) dia bed is shown in Fig. 2. This particular unit will process 8760 litres (2310 gal)/hr of stainless steel pickle liquor. Because of its compact size, the unit can be shipped fully assembled and pretested so that installation and start-up costs are minimal. The basic mechanics of the system are simple. Consequently, reliability is high and maintenance costs are low. For most pickling applications, the system is operated in a bypass arrangement directly on the pickling tank (Fig. 3). Operation in this manner maintains the tank at a consistent, low level of metal contamination. Through regular bath analysis and acid makeup, the acid concentration can also be held at a constant value. n this way, the pickling process can be optimized. Contaminated acid flows through a filter directly to the unit. The acid is retained in the unit and the metal-bearing by-product or waste stream exits from the unit. The waste stream flows from the unit on a near continuous basis throughout the day. For the plant waste treatment system, a small continuous flow such as this is much easier to handle than the large instantaneous load that is normally generated when a whole bath is dumped. Water is used to elute the acid from the unit and this acid product flows directly back to the process tank. Regular additions of concentrated makeup acid are required to replace acid neutralized through metal dissolution. Where several process tanks are in use, the system may be used on all tanks continuously or in rotation. As a rule of thumb, it is possible to easily remove close to 60% of the metal from the acid in one pass and recover approximately 90% of the free acid. Although it is usually feasible to achieve a 90% separation by adjusting operating conditions, production of high purity acid is usually not necessary. 1 F Fig. 3 - Typical APU installation configuration. )WASTE Typical results for steel and stainless steel pickling are shown in Table and 11. Removal of suspended solids from the acid prior to processing is essential. Depending on the nature of the solids and the acid solution, a variety of filters have been employed. Excellent success has been achieved utilizing a depth filter similar to the multimedia sand filter employed in water treatment facilities. Sulfuric acid pickling Sulfuric acid is usually limited to batch pickling operations, although there are still some continuous sulfuric pickling lines in operation. To achieve satisfactory pickling rates it is necessary to operate sulfuric acid pickle baths hot. This does not present a problem for the APU since the resins are stable up to 100 C. Systems have been in continuous service at 80 C in excess of four years with no significant drop in performance. Appreciable improvements can be achieved in terms of improved productivity, acid savings and pollution abatement with the APU. Productivity improvements - Generally, at least 10% sulfuric acid must be maintained in the pickling bath to insure minimum pickling rates. The acid becomes spent when the dissolved iron level reaches 6 to 8% by weight. At higher levels, the sulfate will crystallize out of solution. Care must be taken to not allow either the free acid level to get too high or the bath temperature too low, as this will depress the solubility of the ferrous sulfate and cause crystallization. The Flg. 2 - Typical APU. TABLE Acid recovery Concentrations Pickling Feed, Product By-product, application Component g/iitre g/iltre g/iitre Sulfuric acld H2SOs Fe Hydrochloric acid HC Fe TABLE 1 Typical APU field results: stalnless steel plckllng Composition Fe, gl Free HF, Free HN03, Relative APU stream litre Normal Normal flow rate Feed Product Bv-Droduct iron and Steel Engineer January 1990

5 ~~ sulfate crystals tend to clog transfer pipes and pumps, interfering with bath replacement schedules. Cleanout is time consuming, often resulting in appreciable, unscheduled line downtime. Continuous bath purification with an APU insures that dissolved iron concentrations do not exceed solubility limits, so that downtime due to bath replacement, aggravated by crystallization problems, is eliminated. Pickling rates in sulfuric acid are strongly dependent on both free sulfuric acid content and dissolved iron levels. Since the APU recovers free (ie, unused) acid in solution, higher sulfuric acid concentrations can be used economically on a continuous basis. This can lead to significantly increased productivity levels. A plant using a batch sulfuric acid pickle typically formulates a fresh bath with 15% w/w (165 ghitre) fresh acid. Spent pickle liquor is often mixed with the virgin acid to provide some iron (1 to 2%) in solution, limiting the aggressiveness of the acid. Spent baths usually contain 8 to 10% free acid and 7 to 8% dissolved iron (typically [HzS04] = 110 ghitre, [Fe] = 80 ghitre). These bath compositions can be related to minimum pickling time (Fig. 4).3 The heavy line shown in Fig. 4 represents the operating line of the pickle bath. t shows the pickling time to vary between 56 and 103 s. The average time (point A) is 80 s. The operator would have to continually adjust pickle times to avoid either under or over-pickling. An APU system can continuously maintain a pickle bath at 15% free acid and 4 to 6% dissolved iron. Pickling times within this range (point B) are 62 to 64 s. This represents an average improvement of 16 to 18 s or approximately 20%. Thus, for example, a 6-day work week could be shortened to five days. f 20% free acid is used, pickling times can be reduced to 52 s. n this case, a 24-hr/day operation could eliminate one shift per day. n addition to improving productivity, the system will also stabilize the pickling operation. Pickling rates are constant when the acid and iron levels are controlled which makes it easier for line operators to insure that uniform, high-quality material is produced. Acid savings - f the amount of metal dissolved is 0.5% of the material processed and the loss of pickling solution to Fig. 4 - Pickling times in sulfuric acid.3 120, 100 i" - m f 80 K _ (1 8) (37) (55) (7 4) (9 21 Concentration Of Ferrous Sulfate, YO (Fe %) dragout is neglected, the amount of 93% sulfuric acid required for batch pickling steel with and without an APU can be calculated as and tons acidlton metal pickled, respectively. Thus, a reduction in sulfuric acid consumption of more than 33% can be expected. Pollution abatement - The alkali (either caustic soda or lime) required to neutralize spent pickle liquor is proportional to the acid consumption. Thus, by employing an APU system, neutralization chemical costs can also be reduced by a factor of approximately one-third. Lime neutralization generates large amounts of calcium sulfate (gypsum) sludge in addition to iron hydroxide. Neutralization with caustic soda reduces solid waste but is usually prohibitively expensive. The amount of sludge generated by lime neutralization with and without an APU, assuming the dewatered sludge contains 70% water, can be calculated as and tons acidlton metal pickled, respectively. ron hydroxide, Fe(OH)2, with or without APU is tons/ton metal pickled. Thus, the amount of iron theoretically dissolved and the resulting sludge is the same regardless of whether the system is employed. n practice, however, the better control that the process affords helps eliminate over-pickling, tending to reduce the amount of iron hydroxide produced as well as the amount of acid consumed. Total sludge without APU is = tons/ton metal pickled and with APU = = tons/ton metal pickled. The amount of solid waste generated is, therefore, reduced by 24%. n summary, the use of an APU on a sulfuric acid pickling line can accomplish the following: Elimination of downtime to dump and replace spent pickle liquor. Elimination of shock loading on the waste treatment system that occurs when a spent bath is dumped. Elimination of clogged pipes and pumps with ferrous sulfate crystals. Consistent pickle liquor composition and pickling performance with a 20% increase in productivity. Reduce sulfuric acid purchases by one-third, alkali purchases for neutralization by 34% and amount of solid waste generated by 24%. Hydrochloric acid pickling Many pickling operations have converted from sulfuric acid to hydrochloric acid because of the increased speed and superior surface finish produced. Despite the advantages of HCl, others have not converted because HC regeneration systems were too expensive and complex and fume control can be problematic. The availability of the APU now makes this option viable. While the pickling speed of HCl baths is insensitive to iron concentration, it is strongly dependent on hydrochloric acid concentration and temperature. For this reason, it is common to continue to operate the pickling baths on continuous strip lines until the iron concentration well exceeds 100 g/ litre. Upon initial consideration, there would seem to be little benefit in utilizing an APU to control iron contamination on a continuous HC1 pickling line. Closer examination, however, reveals a number of potential benefits including reduction of toxic HC1 fumes, productivity improvements and energy savings. Fume reduction - One of the major disadvantages of HCl is its strong tendency to fume. The amount of fuming increases rapidly with HC1 concentration and temperature. January 1990 ron and Steel Engineer 57

6 ~~ Although not widely recognized, the concentration of iron chloride in the solution also greatly affects fuming. Equation (1) relates the concentration of HCl fumes over a pickle tank to acid concentration, iron concentration and temperat~re.~ log [HCl] (gas) = ' [HCl] (solution) [FeClz] (solution) (1) where [HCl] (gas) = fume concentration, g/cu metre of inert gas. [FeClz] (solution), [HCl] (solution) = concentration in pickle solution, weight %. T = temperature of pickle tank, "C. The following empirical equation has been developed to estimate the descaling time required for hot-rolled low carbon steel Log t = log [HCl] + (T + 273) where t = descaling time, s [HCl] = concentration of hydrochloric acid, gflitre T = temperature of pickle tank, "C Assuming that a satisfactory pickle of steel strip in 30 s is desired, with a bath containing [HCl] = 45 gflitre and [Fe] = 100 gflitre [HCl] = 3.74% and [FeCl] = 18.9%), a temperature of 95 C can be calculated from equation (2). Under these conditions, a fume concentration of 5.06 g/cu metre can then be determined from equation (1). Assuming the solution is discharged continuously to maintain this composition, a loss of 0.45 g of HCl/gram of dissolved iron in the liquid waste would result. Through use of an appropriately sized APU, it is possible to operate the pickle tank at [HCl] = 100 gflitre and [Fe] = 50 gflitre ([HCl] = 8.82%, [FeC12] = 10%) while losing approximately the same quantity of acid in the waste. At this composition, the bath operating temperature could be reduced to 70 C and still achieve a 30-s pickle. The resulting fume concentration would be reduced to 2.05 g/cu metre, a 60% reduction. Productivity improvements - f fume reduction is not a priority, productivity can be improved without adversely increasing the waste problem. By operating the bath at [HCl] = 100 gflitre and [Fe] = 50 gflitre ([HCl] = 8.82%, [FeC12] = 10%) and maintaining a temperature of 95"C, the pickling time can be reduced to 15 s, a 100% increase in productivity. While the fuming losses would increase (12.1 g/cu metre), the acid losses in the waste would be the same as if no APU were utilized. Many HC1 pickling operations operate a series of two or more countercurrent pickle tanks. The lead tank is dumped periodically and the content of each of the other tanks is transferred to the next tank. This procedure is effective in maximizing acid utilization and minimizing waste. However, it is labor intensive and results in an appreciable amount of nonproductive time. For example, one batch wire pickler operates two HC1 pickle tanks. Wire bundles are placed in tank No. 1 for 24 min, then removed and placed in tank No. 2 for an additional 24 min. Weekly, the contents of tank No. 1 are pumped out and hauled off-site by tanker. Tank No. 1 is cleaned of sludge and the contents of tank No. 2 are transferred to tank No. 1. Tank No. 2 is then made up with fresh HCl and city water. The downtime for this process is approximately 4 to 6 hr. Use of an APU would eliminate this labor and downtime by extending bath life indefinitely. Field tests at the same installation have shown that a satisfactory pickle can be obtained in a single-stage, 23-min pickle with a bath containing HC1= 222 g/litre and [Fe] = 35 gflitre. An APU makes such a bath composition feasible without paying a penalty for waste disposal. This would result in the elimination of one pickle tank, a 100% increase in productivity, plus an additional 4 hr of productive time which is presently lost when the tanks are dumped and cleaned. Energy savings - The trend in continuous HCl pickling has been to increase temperatures to allow higher pickling rates at lower acid concentrations. With an APU, it is possible to utilize relatively high acid concentrations to obtain satisfactory rates at lower temperatures. For example, to achieve a 30-s pickle at [HCl] = 45 gflitre and [Fe] = 100 g/ litre the temperature must be 95 C (203"F), while the temperature can be reduced to 63.5"C (146 F) if the bath is maintained at [HCl] = 130 gflitre and [Fe] = 50 gflitre. The main energy loss from a pickle tank is due to evaporation of water from the surface of the solution. Based on data presented by Rausch: evaporation from a pickling bath can be expressed by the following equation when the air velocity across the top of the tank is 0.5 metre/s: log E = T (3) where E = evaporative rate, litres/hr/sq metre T = solution temperature, "C The evaporation rates at 95 C and 63.5"C are calculated as 43.5 and 6.84 litres/hr/sq metre, respectively. The reduction in temperature would then result in an 84% reduction in evaporative losses with a corresponding reduction in energy requirements for heating the pickle bath. Fume losses (4.092 g/cu metre) would be approximately 19% lower by operating with the APU at the lower bath temperature, despite the higher acid concentration. The various options for operating an HC1 pickling line with and without an APU are summarized in Table 11. Depending on the individual circumstances, a unit can often be justified on continuous hydrochloric acid pickling lines on the basis of the following potential benefits: Elimination of downtime to dump and replace spent pickle liquor. A 60% reduction in HCl fumes or a 100% increase in line productivity or an 84% reduction in energy consumption. Stainless steel pickling Mixtures of nitric and hydrofluoric acids are used to remove scale from most grades of stainless steel. During the pickling - TABLE 111 HC pickling conditions Bath composition Plckiing Temperature, HC, Fe lime, Fumes, Evaporation, System Requirement "C g/iitre g/litre S g/cu metre litres/hr/cu metre No APU With APU Faster pickle With APU Less fumes With APU Less energy ron and Steel Engineer January 1990

7 process, iron, chromium and nickel are dissolved in the acid. Generally, these baths are formulated with 10 to 15% by weight nitric and 1 to 4 % hydrofluoric acid. Bath temperatures are maintained at 55 to 65 C. APU resins are susceptible to strong oxidation in hot nitric acid. For this reason, the pickle liquor must be cooled to less than 32 C (90 F) before treatment. PTFE-filled graphite or PVDF heat exchangers are employed for this purpose. Concentrated nitric and hydrofluoric acid must be added regularly to maintain free acid levels. However, when the metals content of the solution exceeds 5 to 6%, metal salts begin to crystallize out of solution and the bath must be dumped. For example, one particular continuous stainless steel strip annealing pickling line is shut down for several hours once every 10 to 15 days to dump one 29,000-litre (7700 gal) pickle tank. Approximately 5 cu metres of sludge must be manually shoveled from the tank each time. This task is extremely unpleasant and hazardous for plant person n e 1. By utilizing an APU, metals can be continuously removed from the pickle tank, so that precipitation of metal salts does not occur. The filtration system in the unit continuously removes inert solids from the pickle liquor and, by making regular acid additions to the tank, the life of the pickle solution can be extended indefinitely. Production downtime to dump the bath and clean the tank, is eliminated along with the associated labor. Pickling rates vary linearly with the free HF concentration in the pickle liq~or.~ With an APU it is possible to maintain a higher free HF concentration in the bath (to decrease pickle times), without significantly increasing the consumption of hydrofluoric acid. Case study - A case study recently performed at a batch stainless steel tube pickling operation in Markham, Ont., illustrates the benefits derived.s Prior to installation of a unit, it was common practice to dump a 20,000-litre (5280-gal) pickling tank and recharge the tank with fresh acid feed at 12 to 14-week intervals. Throughout these intervals, pickling times would increase as the active concentration of hydrofluoric acid decreased with increased dissolved metals. Additional hydrofluoric acid was added occasionally when the average pickling time exceeded 2 hr. Some nitric acid was also added to top up the level of the bath. n an effort to reduce the pickling times, the bath temperature would sometimes be raised to 40 to 50 C. Unfortunately, the increased bath temperature resulted in increased evaporative acid losses and objectionable, unsafe working conditions around the pickling tank. Generally, the average pickling time per charge was approximately 84 min over the life of the bath. ncidents of under and over-pickling were sometimes encountered. These were due not only to the decreasing acid activities over the life of the bath, but also because reliable acid and iron analyses were not available to determine when the bath needed replenishment or dumping. (Proper bath monitoring techniques should be considered as part of the application of any acid purification system.) A considerable amount of time was, therefore, invested in developing new techniques and refining established techniques for bath analysis. The dissolved iron level would rise typically to above 45 g/ litre by the time the bath was dumped. And, as discussed previously, significant ferric fluoride sludges tend to form in the pickling baths at such high dissolved iron concentrations. n fact, extensive cleaning of the tank was usually needed at dump time to remove the accumulated sludge from the tank bottom. These sludges might also act to reduce the hydrofluoric acid bath activity over long time intervals. n Nov. 1988, a few months after the APU was installed, the optimum pickling bath operating parameters were determined and adopted. Based on concerns for a healthier working environment and improved bath heater operation and maintenance requirements, bath temperature was set at approximately 28 C. Typical results are shown in Table 11. Although analysis of chromium and nickel are not shown, other tests have confirmed that these metals are removed in proportion to the iron. These data show that the APU is recovering 79% of the free hydrofluoric acid and 95% of the free nitric acid, while removing 89% of the metal contamination. The present average pickling time, with the unit installed, is consistently close to 55 min and it is no longer necessary to increase the bath temperature to compensate for reduced pickling rates. Acid losses due to fuming and the associated health and safety hazards have been eliminated. Generation of sludge in the pickle tank has been dramatically reduced, thereby eliminating periodic tank dumps with the associated production downtime and labor requirements. Pickling quality is consistently better. Recommended analytical methods were implemented. As a direct consequence of the reduced average pickling time and improved surface finish consistency, the pickling line production rate improved by 60% on a total length treated basis, or by 45% of a total weight basis. Plant management believes that there is potential to reduce the pickling times further, to approximately 45 min, thereby further increasing the pickling production rate. Total acid consumption was reduced by 71% during the first five months of operation, despite the fact that the faster pickling times resulted in 60% more production. n addition, this evaluation was done during the initial period during which familiarization with the system and the new analytical techniques were taking place. t is expected that the 1989 figures will show further significant improvements. Pollution abatement - Nitrate and fluoride ions are difficult to treat and are considered pollutants. With environmental laws becoming increasingly strict, the reduction in nitrate and fluoride levels in the final effluent is also an important benefit. Reduction in nitrate and fluoride levels are commensurate with the reduction in purchases of the acids. An APU installed in a stainless steel pickling line can offer the following benefits: Consistent pickle liquor composition and pickling performance. Decreased pickle times and increased productivity. Elimination of downtime to dump and replace spent pickle liquor. Elimination of labor for removal of scale and crystals on the bottom of the pickle tank. Elimination of shock loading on the waste treatment system that occurs when a spent bath is dumped. Reduced purchases of nitric and hydrofluoric acids and resulting effluent levels. Reduced operating temperatures, fuming and energy consumption. Summary An acid recovery system based on the APU can offer substantial benefits to pickling operations through Pollution abatement. Resource recovery. Energy conservation. ncreased productivity. Reduced line size. The simplicity, small size and low cost of the unit make January 1990 ron and Steel Engineer 59

8 recovery feasible, even in cases where conventional regeneration systems have been previously considered and rejected. REFERENCES 1. Brown, C. J., Acidmetal Recovery by Recoflo Sorption, Roc. 23rd Conference of Metallurgists of the CM, Quebec City, Canada, Aug Brown, C. J., Treatment of Plating Wastes, Metals Handbook, Ninth Ed., Vol. 5, Surface Cleaning, Finishing and Coating, American Society for Metals, pp ASM Committee on Pickling of ron and Steel, Pickling of ron and Steel, Metals Handbook, Ninth Ed., Vol. 5, Surface Cleaning, Finishing and Coating, American Society for Metals, p Hudson, R. M., and Warning, C. J., Minimizing Fuming during Pickling with Hydrochloric Acid, Sheet Metal ndustries, June Hudson, R. M., and Warning, C. J., Factors nfluencing the Pickling Rate of Hot-Rolled Low-Carbon Steel in Sulfuric and Hydrochloric Acid, Metal Finishing, June Rausch, W., Die Phosphatierung Von Metallen, Eugen Lenze Verlag, Saulgau, Wurtenberg, BRD, p Covino, Jr., B. S., Fundamentals of Stainless Steel Acid Pickling Processes, U.S. Bureau of Mines nformation Circular 9195, Eco-Tec Nitric/Hydrofluoric Acid Purification Units Saves Money and Permits Simpler, More Consistent Pickling of Stainless Steel Products, Eco-Tec nc., Engineering Bulletin No. 89-1, Jan A - 60 ron and Steel Engineer Printed in USA. 1000/2/90