RECOVERY OF STAINLESS STEEL PICKLE LIQUORS: PURIFICATION VS. REGENERATION

Transcription

1 RECOVERY OF STAINLESS STEEL PICKLE LIQUORS: PURIFICATION VS. REGENERATION CISA International Steel Congress, September,2002 Beijing China Craig J. Brown, P. Eng., Eco-Tec Inc., Pickering, Ontario, Canada Pickling Chemistry During finishing of stainless steel, the metal is typically rolled and then annealed to achieve the desired structure and material properties. Because annealing is carried out in the presence of air, a film of oxide consisting mainly of chromium oxide (Cr 2 O 3 ), ferrous oxide (FeO), and manganous oxide (MnO) forms on the surface of the steel. The Cr 2 O 3 content is normally between 55 and 65%. Normally, the film of oxide is fairly porous and contains small cracks. A zone that is depleted of chromium is normally formed under the oxide film. A process called pickling is used to clean and condition the surface of the metal after annealing. The pickling of stainless steel requires three distinct processes. The first is removal of the thermally grown scale for appearance purposes and to facilitate cold working of the steel. The second process increases the corrosion resistance of the final product by dissolving the chromium-depleted zone that generally forms during annealing. During the third process, a minimum amount of bulk steel is dissolved, giving the desired brightening effect to the final product. The first, scale removal step is often accomplished using shot blasting or electrolyte pickling in neutral salt. Recently, sulphuric acid pre-pickles have been commonly employed for scale removal. The remaining two pickling steps are normally carried out in mixed acid solutions which typically contain g/l nitric acid and g/l hydrofluoric acid. The nitric acid is a strong oxidizing agent and oxidizes the metals and metal oxides, forming Cr 3+, Fe 3+ and Ni 2+ ions. Stable complexes are formed between these metals (particularly the Cr 3+ and Fe 3+ ) and the hydrofluoric acid. The reaction between nitric acid and the metal converts the nitric acid into nitrous acid (HNO 2 ) or dissolved NO x. Although the actual chemical equilibria and stoichiometry of the pickling reaction are quite complex, the basic reaction can be expressed by equation (1). Complex mathematical models have been developed which more accurately model the reaction and the speciation of the participating chemicals 1. 4Fe + 8HF + 4 HNO 3 6HNO 2 + 4FeF NO H 2 O (1) The rate of pickling is principally dependent on the pickling bath temperature, the free hydrofluoric acid concentration and the dissolved iron and chromium concentrations 2. A distinction needs to be made between the terms total acid concentration and free acid concentration. The total acid concentration represents the amount of acid present before it is reacted with any metal. As a result the so-called total hydrofluoric acid concentration of a spent pickle liquor includes all of the fluorine in the solution i.e. fluoride that is complexed with the metals (eg. FeF 2 + ) as well as the fluoride that is bound with the hydrogen ions (HF) and any free fluoride ions (F - ) that exist in the solution. The free hydrofluoric acid concentration of the spent liquor includes Sales & Marketing TP /2002

2 only the HF and F - species which are still available to react with more metal. In order to maintain an acceptable pickling rate, a significant concentration of free nitric acid and hydrofluoric acid must be maintained in the solution. Concentrated nitric and hydrofluoric acid can be added to maintain free acid levels. However, when the metals content of the solution exceeds g/l, metal salts begin to crystallize out of solution and the bath must be dumped and replaced. Crystallizing conditions are avoided if possible, since it is a very time consuming, messy and hazardous task to regularly drain the pickling tubs and clean the strongly adhering sludge from the tank. Ideally, the metals should be maintained at a concentration of g/l to avoid crystallization. Typical spent and preferred working pickle liquor compositions are shown in Table 1 and Table 2. Total nitric and hydrofluoric acid concentrations have been calculated based upon the specified free acid and metal concentrations, using a computer program called SPEC developed by the U.S. Bureau of Mines 1. The stainless steel pickling process generates a considerable quantity of spent pickle liquor containing large quantities of nitrate, fluoride and heavy metals. While a basic waste treatment system employing lime neutralization is capable of removing most of the fluoride and metals, nitrate removal is more problematic. The most common technique for nitrate removal is biological treatment. Since heavy metals are toxic to the biological growth used in this process, it must be conducted in a separate treatment system. Table 1: Typical Spent Pickle Liquor Composition Free nitric acid = 150 g/l Total nitric acid = 242 g/l Free hydrofluoric acid = 30 g/l Total hydrofluoric acid = 65.8 g/l Total metal = 60 g/l ( calculated values) Table 2: Preferred Working Pickle Liquor Composition Free Nitric acid = 150 g/l Total nitric acid = 196 g/l Free Hydrofluoric acid = 30 g/l Total hydrofluoric acid = 48 g/l Total metal = 30 g/l ( calculated values) Sales & Marketing TP /2002

3 Even if it is possible to satisfy environmental concerns with a welldesigned waste treatment process, the chemicals lost in the spent pickle liquor represent a potential major financial asset if they could be effectively recovered. Rinsing After the stainless steel has been pickled, the adhering acid must be rinsed from the metal surface. This is typically accomplished by passing the strip through a series of counter-currently flowing water rinse tanks. The spent rinsewater contains dilute pickle liquor with considerable levels of nitric acid, hydrofluoric acid and metals. It must therefore also be treated in a suitable waste treatment system prior to sewer discharge. While the quantity of chemicals lost to rinsing is appreciably less than that lost in the spent pickle liquor itself, it is not insignificant. Fuming Based upon equation (1), approximately 1 kg of nitric acid is consumed in the pickling process for each kg of metal dissolved. This is borne out by practical experience. The nitrous acid that is generated (HNO 2 ) is in equilibrium with NO x (NO and NO 2 ), which can escape to the atmosphere according to equation (2) and ends up in the fume exhaust. The NO x emissions from a steel pickling tub typically contain approximately equal amounts of NO and NO 2. The limited solubility of nitrous acid shifts the equilibrium towards release of NO x to the vapor phase and subsequent escape to the atmosphere. 2 HNO 2 NO + NO 2 + H 2 O (2) Control of fuming from mixed acid pickling baths is a major environmental issue. Various techniques are employed including: 1. Water scrubbing 2. Caustic scrubbing 3. Urea addition 4. Selective catalytic reduction 5. Hydrogen peroxide addition Because of the difficulty in controlling NO x, emissions, a considerable effort has been invested by the industry in developing alternative pickle bath chemistries that do not use nitric acid. The Cleanox process from Henkel for example, uses a bath containing sulphuric acid, hydrofluoric acid and peroxide, totally eliminating nitric acid. Other, similar formulations are currently being developed and evaluated 3,4,5,6. EXHAUST FUMES SCRUBBER TO WASTE TREATMENT DRAGOUT SS STRIP PICKLE TANK RINSE TANK SPENT PICKLE LIQUOR TO WASTE TREATMENT FRESH ACID MAKEUP Figure 1: Typical Pickling Process RINSE TO WASTE TREATMENT Sales & Marketing TP /2002

4 Table 3: Typical Mass Balance no recovery Nitric Acid Losses (kg/t metal pickled) nitric acid to fume exhaust = 1 kg/kg metal x 2 kg/t = 2 kg/t nitric acid to dragout = 0.2 L/m 2 x 95 m 2 /T x 242 g/l / 1000 g/kg = 4.60 kg/t nitric acid to bath bleed = 2 kg/t x 1000 g/kg /60 g/l x 242 g/l / 1000 g/kg = 8.07 kg/t Total nitric acid consumed = kg/t Hydrofluoric Acid Losses (kg/t metal pickled) hydrofluoric acid to dragout = 0.2 L/m 2 x 95 m 2 /T x 65.8 g/l / 1000 g/kg = 1.25 kg/t hydrofluoric acid to bath bleed = 2 kg/t x 1000 g/kg /60 g/l x 65.8 g/l / 1000 g/kg = 2.19 kg/t Total hydrofluoric acid consumed = 3.44 kg/t Chemical Mass Balance An acid mass balance for a typical stainless steel pickling operation (see Figure 1) with no recovery system is shown in Table 3. This mass balance is based on the following assumptions: Amount of stainless steel processed: 1 T (1000 kg) Metal dissolution rate: 0.2% Surface area of metal strip: 95 m 2 /T Dragout rate: 2 litresperm 2 It can be seen from Table 3 that up to (8.07/14.67 x 100%)= 55% of the total nitric acid and (2.19/3.44 x 100%)= 63.6% of the total hydrofluoric acid that is consumed is potentially recoverable from the spent pickle liquor bleed. Purification vs Regeneration Spent pickle liquor contains a mixture of the residual unreacted free hydrofluoric and nitric acid as well as metal salts of those acids. By removing the metal fluoride and nitrate salts and replenishing the free acid concentrations with fresh acid, it is possible to extend the bath life indefinitely. This is the basis of operation of a so-called purification system. A purification system will never totally eliminate the purchase of fresh acid. It will however, substantially reduce the quantity that is consumed. In addition to recovering the free acids, a regeneration system is designed to convert the metal salts back to free hydrofluoric and nitric acid, thereby rejecting the metal impurities typically as oxides. Manufacturers imply that it is possible to totally eliminate or dramatically reduce the purchase of hydrofluoric and nitric acid with a regeneration system. In actual practice, this is not the case however, as shown below. Sales & Marketing TP /2002

5 Figure 2: APU Purification System The APU System The two principle processes developed for purification of mixed stainless steel pickle liquors are acid retardation (i.e. ion exchange) and diffusion dialysis (DD). Acid retardation is by far the most widely used system because of its low cost, simplicity, reliability and superior performance 7. Eco- Tec alone has installed hundreds of its APU ion exchange systems in 30 different countries in a variety of acid purification applications since the process was introduced in Over 70 stainless steel pickling operations have installed APU systems. It is estimated that fewer than 10 DD systems have been installed on mixed acids. In fact, a number of these systems have been replaced with APU s over the past few years. The APU system utilizes an ion exchange resin that has the ability to adsorb strong acids from solution while excluding metallic salts of those acids. The process is reversible in that the acid can be readily desorbed from the resin with water. It is thus possible, by alternately passing contaminated acid and water through a bed of this resin, to separate the free acid from the metal. The nature of the process is that only very small volumes of solution can be processed each cycle. The challenge then, is finding a way to efficiently elute purified acid from the resin without contaminating it with the spent acid feed and without excessively diluting the purified acid. A patented 8 ion exchange technique called Recoflo proved to be ideally suited to just such an application. Through the use of short (60 cm), tightly packed beds of fine mesh resins and countercurrent regeneration, Recoflo provides the necessary tool with which to achieve the required separation efficiency. There are two steps in the APU process - the upstroke and the downstroke. During the upstroke, contaminated acid is pumped into the bottom of the resin bed. Acid is sorbed by the resin particles and the remaining de-acidified metallic salt solution, called the byproduct, is collected from the top of the bed. Next, during the downstroke, 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 about 5 minutes to complete. Nitric acid is employed in pickling stainless steel because it is a strong oxidizing agent. Unfortunately, it also has a tendency to oxidize ion exchange resin. If the acid is too concentrated or too hot, the reaction can be violent, causing explosion under some circumstances 9. To avoid resin degradation, the pickle liquor must be cooled before it is fed to the ion exchange unit. It has been found that an APU resin in mixed Sales & Marketing TP /2002

6 acid service can operate safely and remain effective for more than seven years, providing the feed is cooled to less than 32 C before treatment 10. A typical APU system is shown in Figure 2. Pickle liquor is pumped directly from the pickle tub through a heat exchanger that cools the acid to less than 32 C. The cooled solution is then pumped through a filter to remove suspended solids. The filter is a backwashable depth media filter. The filtered acid is collected in a tank and then fed to the APU. The free hydrofluoric and nitric acids are sorbed by the APU resin bed and a de-acidified metal salt byproduct solution is collected from the top of the resin bed and discharged to the plant waste treatment system along with the rinsewater and scrubber water. Water is then pumped into the top of the resin bed, eluting acid from the resin. A purified acid product is then recycled directly back to the pickle tank. The byproduct/product cycle repeats every few minutes. Scaleup is accomplished by increasing the diameter of the APU resin bed. APU's are constructed in a range of capacities. Small units, called Micropur (bed diameters from 15 cm (6 inches) to 50 cm (20 inches) utilize hydro-pneumatic reservoirs to pump the feed acid and elution water through the resin bed. Larger units (up to 274 cm (108 inches)) utilize electronic measurement of flows and are equipped with external holding tanks and pumps. Because of its compact size, the APU can be shipped fully assembled and pre-tested. As a result, installation and startup costs are minimal. The basic mechanics of the system are very simple, consequently reliability is high and maintenance costs are low. EXHAUST FUMES SCRUBBER TO WASTE TREATMENT SS STRIP PICKLE TANK RINSE TANK ACID COOLER MEDIA FILTER BYPRODUCT TO WASTE TREATMENT RINSE TO WASTE TREATMENT FEED TANK APU PURIFIED ACID PRODUCT Figure 3: APU Purification System Sales & Marketing TP /2002

7 Table 4: Typical APU Performance Feed (bath) (g/l) Product (g/l) Byproduct (g/l) Free [HNO3] Total [HNO3] Free [HF] Total [HF] Total metal ( calculated values) While it is possible to treat spent pickle liquor at the composition shown in Table 1, it is preferable to operate the pickling process at a much lower metal concentration. This provides more consistent quality and shorter pickling times, without increasing acid consumption. This is one of the major advantages of a purification system. In addition, since the metal concentration is lower, the total nitric and hydrofluoric acid concentrations are reduced. This reduces the loss to dragout. If the pickle bath is operated at the preferred composition shown in Table 2, the composition of the product and byproduct streams produced by an APU would be as shown in Table 4. The total acid concentrations were calculated from the stated free acid and metal concentrations using the Bureau of Mines computer program 1. The mass balance for this pickling operation is shown in Table 5. It can be seen that the APU recovers over 98% of the free nitric acid and 80-90% of the total nitric acid while removing 70% of the metals. Free hydrofluoric acid recovery is over 93% and total hydrofluoric acid recovery is 60-70%. Based upon this performance, the total consumption of nitric acid can be reduced from to 7.26 kg/t, which represents a 50% reduction, compared to the case of no recovery, shown in Table 3 (Table 1 bath composition). The total hydrofluoric acid consumption would be reduced from 3.44 to 2.78 kg/t, which is a 19.2% reduction. Along with these chemical savings are benefits associated with operating the bath at the preferred, lower metal concentrations shown in Table 2. These benefits include faster, more consistent pickling speed, better quality and elimination of crystallization. Roaster The spray roasting process has been successfully used for many years for regeneration of hydrochloric acid carbon steel pickle liquors. Ruthner adapted the spray roasting process to mixed acids in Since that time the process, called Pyromars, has achieved limited acceptance. It is estimated that about 6 of these systems have been sold. It is interesting to note that three of these mills have chosen purification systems over additional roasters for subsequent requirements. The Pyromars roaster process is shown in Figure 4. The process basically operates in the following way: Waste pickle liquor from the pickling operation is pumped from the pickling line to the waste acid tank. From there it is fed to the Venturi pre-concentrator, where part of the acid evaporates. The pre-concentrated acid is pumped into the spray-roasting reactor, which is a large, direct-fired, refractory lined cylindrical vessel. Sales & Marketing TP /2002

8 Table 5: Typical Mass Balance APU recovery Nitric Acid Losses (kg/t metal pickled) Total nitric acid to fume exhaust = 1 kg/kg metal x 2 kg/t = 2 kg/t Total nitric acid to dragout = 0.2 L/m2 x 95 m 2 /T x 196 g/l / 1000 g/kg = 3.72 kg/t Total nitric acid to APU byproduct = 2 kg/t x 1000 g/kg /21 g/l x 16.2 g/l / 1000 g/kg = 1.54 kg/t Total nitric acid consumed = 7.26 kg/t Hydrofluoric Acid Losses (kg/t metal pickled) Total hydrofluoric acid to dragout = 0.2 L/m2 x 95 m 2 /T x 48 g/l / 1000 g/kg = 0.91 kg/t Total hydrofluoric acid to APU byproduct = 2 kg/t x 1000 g/kg /21 g/l x 19.6 g/l / 1000 g/kg = 1.87 kg/t Total hydrofluoric acid consumed = 2.78 kg/t In the reactor, the acid evaporates and the metal fluorides and nitrates decompose at high temperatures (i.e. 400 C) into metal oxides. These oxides are collected in the reactor s conical base and conveyed pneumatically to the oxide bin. The gases produced by this reaction (water vapour, gaseous HF, HNO 3, NO and NO 2, as well as the combustion gases) are fed via a dust separator into the preconcentrator, where they are cooled by the waste pickle acid. The cooled gases from the pre-concentrator pass into the absorption column, where regenerated acid is formed by condensation. Unfortunately at the operating temperature of the roaster the nitrate tends to decomposes forming nitrous oxides. It is necessary to add hydrogen peroxide to the system to convert NO and NO 2 back into HNO 3 so that a reasonable recovery level of nitric acid can be obtained. The regenerated acid flows to the storage tank and from there to the pickling line. The off-gas from the absorption column is pre-cleaned in the gas scrubber and the NO x content in the remaining off-gas is reduced to nitrogen and water in the catalytic NO x reduction unit by selective reduction with ammonia or urea. The system is kept under negative pressure by means of an exhaust fan, which prevents the acidic vapours from escaping. Sales & Marketing TP /2002

9 EXHAUST FUMES SCRUBBER TO WASTE TREATMENT SS STRIP PICKLE TANK RINSE TANK RINSE TO WASTE TREATMENT SPENT ACID TANK RECOVERED ACID TANK PEROXIDE AIR EXHAUST DUST SEPARATOR GAS SCRUBBER AMMONIA OXIDE BIN SPRAY ROASTER REACTOR PRE- CONCENTRATOR ABSORBER HEAT EXCHANGER CATALYTIC NO X REDUCTION OXIDE SCRUBBER TO WASTE TREATMENT Figure 4: Ruthner Pyromars Spray Roaster Published data by the manufacturer 12 indicates that this system is capable of recovering 97% of the total (i.e. free and complexed) hydrofluoric acid and 85% of the total nitric acidthis of course does not take into consideration fume and dragout losses. To minimize capital and operating costs there is a bias towards maximizing the metal concentration to minimize the flow of spent pickle liquor fed to the system. As discussed above, it is preferable to operate the pickling process at a lower metal concentration (eg. 30 g/l) to avoid crystallization problems and to increase the pickling rate. Operation at this lower metal concentration would increase the flow rate of spent pickle liquor fed to the regeneration system and increase the size and cost of the equipment and its operating cost. In addition, the recovery efficiency of the system would suffer. The loss of nitric acid would be expected to increase proportionally. The quantities of peroxide and ammonia or ureas consumed would also be increased. Sales & Marketing TP /2002

10 Table 6: Typical Mass Balance Roaster recovery Nitric Acid Losses (kg/t metal pickled) Total nitric acid to fume exhaust = 1 kg/kg metal x 2 kg/t = 2 kg/t Total nitric acid to dragout = 0.2 L/m2 x 95 m 2 /T x 196 g/l / 1000 g/kg = 3.72 kg/t Total nitric acid roaster losses = 2 kg/t x 1000 g/kg /30 g/l x 196 g/l / 1000 g/kg x 15% = 1.96 kg/t Total nitric acid consumed = 7.68 kg/t Hydrofluoric Acid Losses (kg/t metal pickled) Total hydrofluoric acid to dragout = 0.2 L/m2 x 95 m 2 /T x 48 g/l / 1000 g/kg = kg/t Total hydrofluoric acid roaster losses = 2 kg/t x 1000 g/kg /30 g/l x 48 g/l / 1000 g/kg x 3% = 0.10 kg/t Total hydrofluoric acid consumed = kg/t A mass balance for a roaster designed to maintain the pickle bath at the preferred composition of Table 2 is shown in Table 6. Based upon this performance, the total consumption of nitric acid can be reduced from to 7.68 kg/t which represents a 48% reduction. Surprisingly, this is actually somewhat less than with the APU, which provides a 50% reduction. The hydrofluoric acid consumption would be reduced from 4.19 to 1.01 kg/t, which is a 76% reduction. This is indeed a significant improvement over the APU, which only offers a 19% reduction in HF consumption. This is mainly an economic tradeoff, since lime neutralization of wastewater in the waste treatment system is a highly effective way of removing residual fluoride. Economics An basic economic comparison is offered in Table 7. This table shows the chemical cost to operate a pickling line processing 75 tons per hour of metal, which corresponds to a metal dissolution rate of 150 kg/h. Three cases are considered: 1. No recovery 2. With an APU 3. With a Pyromars roaster The following assumptions were made for this comparison: For the case of no recovery system, it is assumed that the bath is operated so that a spent pickle liquor of Table 1 is periodically dumped to control metal buildup. Sales & Marketing TP /2002

11 For both APU and roaster cases, it is assumed that pickle liquor is continuously withdrawn at the composition of Table 2 and recovered. The cost of chemicals for pickle bath fume suppression is not included since it is the same for all three cases. The cost of lime to neutralize scrubber waters has been included. Electrical energy to run transfer pumps etc. has been neglected since it is minimal Maintenance costs and manpower requirements have not been included. These could be significant, particularly for the roaster system, whose maintenance costs will be appreciable. The cost of sludge or oxide disposal is not included. Chemical costs are FOB figures in the U.S.A 13. Actual delivered prices will likely be somewhat. The economic evaluation in Table 7 clearly shows that the potential savings are greater with the roaster system, principally because of the higher hydrofluoric acid recovery efficiency. The question is whether the increased savings are sufficient to justify the much higher installed capital cost. In addition, when maintenance costs are fully factored in, the gap will narrow.. Table 7: Annual Pickling Operating Costs ($U.S.) No recovery APU Roaster 100% HNO 3 purchases (kg/y) 7,698,950 3,815,100 4,034,100 HNO 3 cost ($/y@$0.242) $1,863,145 $923,254 $976, % HF purchases (kg/y) 1,807,855 1,458, ,200 HF cost ($/y@$2.04) $3,693,189 $2,980,120 $1,081,080 Ca(OH) 2 purchases (kg/y) 8,318,298 5,433,334 2,614,100 Ca(OH) 2 cost ($/y@$0.77) $640,508 $418,366 $201,285 46% Urea purchases (kg/y) 1,088,889 Urea cost ($/y@$0.154) $167,688 35% Peroxide purchases (kg/y) 777,777 Peroxide cost ($/y@$0.539) $419,222 Natural gas purchases (m 3 /y) 3,305,556 Natural gas cost ($/y@$.15) $495,833 Total yearly operating cost (excluding maintenance) $6,196,842 $4,321,740 $3,341,360 Yearly savings vs No recovery $1,875,102 $2,855,482 Sales & Marketing TP /2002

12 Comparison of APU to Roaster The APU purification system and Pyromars regeneration system represent two very different alternatives to dealing with the wastes generated from stainless steel mixed acid pickling operations. It is necessary to consider a wide variety of different factors before making a decision on which type of system to install. Following are some of these factors. Space requirements The roaster system and its components (eg. roaster vessel, absorption columns etc.) is very large and typically requires a separate building. Sufficient space for a compact APU-type purification system can usually be found next to the pickling line. Installation cost For a roaster, the cost of an additional building and field erection costs, as well as the additional piping to and from the pickle line, must be factored into the total installed capital cost. In addition, because of the nature of this process, installation and startup times can be lengthy. The APU is totally skid-mounted and factory tested. Installation costs are minimal, since it is usually located beside the pickle line and startup can be completed in a few days. Capital cost The installed capital cost of a roaster system is about one order of magnitude higher than that of an APU-type purification system of comparable capacity. Nitrate recovery efficiency Total nitric acid recovery efficiency for the two systems is quite comparable. In the case of the APU the lost nitrate ends up in the byproduct which is readily treated in the waste treatment system. With the roaster system, nitrate degrades to NO x which must be scrubbed from the air exhaust from the system. Removal of NO x from air is difficult and often problematic. This is the major reason to consider non-nitrate chemistries. Once it is removed from the air it must still be treated in the waste treatment system. Fluoride recovery efficiency The fluoride recovery efficiency for the roaster is much better than that for the APU system. This is the major benefit of the roaster. The resulting savings are appreciable as shown in Table 7. These savings must be weighed against the higher maintenance costs associated with the roaster and its other disadvantages, however. Since the fluoride losses from the APU are effectively removed by the waste treatment system, there are no environmental benefits associated with higher fluoride recovery. Corrosion By their very nature, roasters are high maintenance. The high temperatures involved (400 C) necessitate regular and frequent replacement of refractory and metal components. The highly corrosive nature of hydrofluoric acid exacerbates the normal corrosion problems that are seen even with hydrochloric acid regeneration systems. Because the APU system operates at low temperatures, corrosion is much less of a problem and low-cost corrosion resistant thermoplastic components can be employed. Operating costs Operating costs for the roaster include appreciable energy in the form of natural gas, as well as cost for the hydrogen peroxide and ammonia or urea required to operate the system. Energy consumption for the APU is minimal- merely low pressure pumping costs. No chemicals are required to operate the APU system. Because of corrosion and its much greater complexity, maintenance costs for the roaster are far higher than for an APU. Sales & Marketing TP /2002

13 Auxiliary waste treatment It is necessary to have a waste treatment system to treat the APU byproduct. One of the advantages claimed by Ruthner is that it is not necessary to have a waste treatment system if a Pyromars system is installed. This is clearly not the case. The roaster only reduces nitric acid consumption by about 48%. The balance, from rinsewaters and fumes is non-recoverable and reports as waste. Metals and fluorides from rinsewaters must be treated and large quantities of metal are dissolved in the sulphuric acid pre-pickle which must also be dealt with. Typically one third to one half of the total metal removed from the strip occurs in the sulphuric acid pre-pickle. Moreover, it is normally considered necessary to have a back-up waste treatment system for when the roaster is shut-down for scheduled and non-scheduled service. Metal recycle A major potential advantage claimed for the roaster is the ability to recycle the recovered metal oxides back to a reduction furnace. Field reports from the few roaster systems that have been installed on mixed acids indicate that limited or no success has actually been achieved in recycling the recovered metal oxide powder, however This may be at least partially due to the difficulty in feeding fine oxide powders into a reduction furnace. It should also be noted that metal values have also been recovered pyrometallurgically from the metal hydroxide sludges produced from APU/waste treatment systems. Inmetco in Pennsylvania, U.S.A, for example, has been taking in metal hydroxide sludge from stainless steel mills and recovering the metal values for many years. Adaptable to non-nitrate chemistries In order to deal with increasingly strict NO x emission regulations, a trend towards nitrate-free pickling chemistries is emerging. Baths containing sulphuric acid and hydrogen peroxide, which have been shown to provide excellent pickling performance, cannot be handled by a roaster. A roaster installed for treatment of mixed acid would become obsolete if the pickling operation were converted to a nitrate-free process in the future. On the other hand, a number of APU systems originally installed for mixed acids have already been converted with almost no modification, to operate on sulphuric/peroxide chemistry with excellent results. Summary and Conclusion Both APU recovery and Pyromars regeneration systems can reduce nitric acid consumption by about 50%. Hydrofluoric acid savings are significantly greater for the roaster than the APU. An auxiliary waste treatment system is required in both cases. A careful evaluation needs to be done to determine if the increased hydrofluoric acid savings offered by the roaster offset its much higher installed capital cost and numerous other disadvantages. Sales & Marketing TP /2002

14 References 1. Stephenson, J. B., G. L. Horter, and H. H. Dewing. "Iron Removal And The Complexity Of Stainless Steel Pickling Liquors." Iron Control in Hydrometallurgy, ed. J.E. Dutrizac, A.J. Monhemius, Ellis Horwood Limited, Pyromars: Recovery of Mixed Acid in Stainless Steel Pickling Lines, Bulletin A e-89, Andritz Ruthner, Vienna. 13. Chemical Marketing Reporter, Schnell Publishing Company, 258, 21 (November 20, 2000). 2. Covino, B. S. Fundamentals of Stainless Steel Acid Pickling Processes, Bureau of Mines and the American Iron and Steel Institute, Zavattoni, M., U.S. patent 5,785,765, July 28, Henriet, D., U.S. patent 5,690,748, November 25, Japan patent / European patent DE 2,827, Dejak, M, Acid Purification and Recovery Using Resin Sorption Technology - An Update, Presented at AESF/EPA Pollution Prevention Conference(1994).. 8. Brown, C. J., U.S. patent 4,6763,507, June 16, Calmon, C., Explosion Hazards Of Using Nitric Acid In Ion Exchange Equipment, Chemical Engineering, 3, 271(1980) Brown, C.J. Mixed Acid Recovery With The APU Acid Sorption System: An Update, presented at the Cleaner Production Workshop, China Steel Corporation, Kaohsiung, Taiwan, Oct , Karner, W., Wurmbauer, D., Drivanac, K.H., Horn, J., U.S. patent 5,149,515, September 22, Sales & Marketing TP /2002