Productivity Improvements Through Recovery of Pickle Liquors with the APU Process

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Continuous recovery of pickle liquors saves acid and reduces waste treatment expense as well as providing a means of decreasing pickling times and ellml- nating downtimes for bath replacement and tank clean out. Reductions In 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 IRON AND STEEL ENGINEER 0 Copyright, Association of Iron and Steel Engineers

Continuous recovery of pickle liquors saves acid and reduces waste treatment expense as well as providing a means of decreasing pickling times and ellmi- nating 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

PICKLING is the chemical removal of surface oxides or ing metallic salts of those acids. The process is reversible, in scale from steel by immersion in an aqueous acid solution. that the acid can be readily desorbed from the resin with While wide variations are possible in the type, strength and water. It is thus possible, by alternately passing contaminat- temperature of the acid solutions used, sulfuric and hydro- ed acid and water through a bed of this resin, to separate the chloric acids are the most common pickling acids for carbon free acid from the metal. steel. Mixtures of nitric and hydrofluoric acids are generally Unfortunately, only small volumes of solution can be pro- used for stainless steel. cessed each cycle. The difficult part of the process is finding Pickle liquors become contaminated with dissolved met- a way to efficiently elute purified acid from the resin without als through use. As the metal concentration increases, the contaminating it with the impure feed acid and without ex- free acid concentration decreases and pickling efficiency cessively diluting it. drops. Additions of fresh concentrated acid are made from A novel ion exchange technique called Recoflo, which has time to time to rejuvenate the bath but eventually it be- been extensively used for recovery of metals from metal fin- comes spent and must be discarded. Pickling speed varies ishing wastewater,2 has proven ideal for this application. continually throughout the life of the bath and it is difficult Through the use of short resin beds, fine mesh resins, coun- to avoid either under or over-pickling. terflow regeneration and various other features, the Recoflo While most pickle acids are relatively inexpensive, the in- technique provides the necessary tool with which to achieve direct costs associated with pickling go well beyond the cost the required separation efficiency. The resulting system is of the acid consumed. Some of these indirect costs include: called an acid purification unit or APU. There are two steps in the basic APU process-the up- Labor to make up fresh acid. stroke and downstroke (Fig. 1).During the upstroke, con- Labor for removal and disposal of spent acid. taminated acid is pumped into the bottom of the resin bed. Cost of neutralizing chemicals. Acid is sorbed by the resin particles and the remaining de- Ultimate disposal of resulting solid waste. acidified metallic salt solution, called the by-product, is col- Reduction in productivity resulting from the inhibit- lected from the top of the bed. Next, during the downstroke, ing action of dissolved metals. Lost production time that occurs while spent acid is removed and replaced. Flg. 1 - APU operating cycle. Quality control problems due to over and under-pickling as bath composition changes. UPSTROKE Recovery of spent pickle liquors can potentially reduce METALLIC SALT WATER BYPRODUCT (WASTE) many of these costs. I Various techniques have been employed to recover waste 4 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. water A simple, low cost unit, called the acid purification unit reservoir (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, pri- marily in metal finishing applications such as sulfuric acid DOWNSTROKE aluminum anodizing, sulfuric/peroxide brass and copper et- SPENT ACID chants and various aluminum, brass, copper and nickel et- I m chants employing nitric acid.l I Recovery of pickle liquors with the APU has recently at- tracted considerable attention in the steel industry and a large number of units have been installed over the past cou- ple of years for this application.

The APU process water PURIFIED Operating principle - Certain ion exchange resins have ACID PRODUCT the ability to sorb strong acids from solution, while exclud-

January 1990 Iron and Steel Engineer 55 water is pumped into the top of the bed, desorbing the purified acid from the resin so that a purified acid product is 1 F I 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 hy- dropneumatic 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 I I equipped with external holding tanks and pumps. I I A typical unit equipped with a 107-cm (42-in.) dia bed is )WASTE shown in Fig. 2. This particular unit will process 8760 litres (2310 gal)/hr of stainless steel pickle liquor. Fig. 3 - Typical APU installation configuration. 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 Typical results for steel and stainless steel pickling are simple. Consequently, reliability is high and maintenance shown in Table I and 11. costs are low. Removal of suspended solids from the acid prior to pro- For most pickling applications, the system is operated in a cessing is essential. Depending on the nature of the solids bypass arrangement directly on the pickling tank (Fig. 3). and the acid solution, a variety of filters have been em- Operation in this manner maintains the tank at a consistent, ployed. Excellent success has been achieved utilizing a depth low level of metal contamination. Through regular bath filter similar to the multimedia sand filter employed in water analysis and acid makeup, the acid concentration can also be treatment facilities. held at a constant value. In this way, the pickling process can be optimized. Sulfuric acid pickling Contaminated acid flows through a filter directly to the Sulfuric acid is usually limited to batch pickling operations, unit. The acid is retained in the unit and the metal-bearing although there are still some continuous sulfuric pickling by-product or waste stream exits from the unit. The waste lines in operation. stream flows from the unit on a near continuous basis To achieve satisfactory pickling rates it is necessary to op- throughout the day. For the plant waste treatment system, a erate sulfuric acid pickle baths hot. This does not present a small continuous flow such as this is much easier to handle problem for the APU since the resins are stable up to 100°C. than the large instantaneous load that is normally generated Systems have been in continuous service at 80°C in excess of when a whole bath is dumped. four years with no significant drop in performance. Water is used to elute the acid from the unit and this acid Appreciable improvements can be achieved in terms of product flows directly back to the process tank. Regular ad- improved productivity, acid savings and pollution abate- ditions of concentrated makeup acid are required to replace ment with the APU. acid neutralized through metal dissolution. Where several process tanks are in use, the system may be Productivity improvements - Generally, at least 10% used on all tanks continuously or in rotation. sulfuric acid must be maintained in the pickling bath to in- As a rule of thumb, it is possible to easily remove close to sure minimum pickling rates. The acid becomes spent when 60% of the metal from the acid in one pass and recover ap- the dissolved iron level reaches 6 to 8% by weight. At higher proximately 90% of the free acid. Although it is usually feasi- levels, the sulfate will crystallize out of solution. Care must ble to achieve a 90% separation by adjusting operating condi- be taken to not allow either the free acid level to get too high tions, production of high purity acid is usually not necessary. or the bath temperature too low, as this will depress the solubility of the ferrous sulfate and cause crystallization. The

TABLE I Acid recovery Flg. 2 - Typical APU. Concentrations Pickling Feed, Product By-product, application Component g/iitre g/iltre g/iitre

Sulfuric acld H2SOs 150 137 9.9 Fe 50 14.3 29.9 Hydrochloric acid HCI 140 142 10.3 Fe 45.6 33.5 20.4

TABLE I1 Typical APU field results: stalnless steel plckllng Composition Fe, gl Free HF, Free HN03, Relative APU stream litre Normal Normal flow rate

Feed 27.0 0.83 1.59 1.00 Product 3.0 0.65 1.51 1.00 Bv-Droduct 20.0 0.15 0.08 1.20

56 iron and Steel Engineer January 1990 sulfate crystals tend to clog transfer pipes and pumps, inter- dragout is neglected, the amount of 93% sulfuric acid re- fering with bath replacement schedules. Cleanout is time quired for batch pickling steel with and without an APU can consuming, often resulting in appreciable, unscheduled line be calculated as 0.0112 and 0.0169 tons acidlton metal pick- downtime. led, respectively. Continuous bath purification with an APU insures that Thus, a reduction in sulfuric acid consumption of more dissolved iron concentrations do not exceed solubility limits, than 33% can be expected. so that downtime due to bath replacement, aggravated by crystallization problems, is eliminated. Pollution abatement - The alkali (either caustic soda or Pickling rates in sulfuric acid are strongly dependent on lime) required to neutralize spent pickle liquor is propor- both free sulfuric acid content and dissolved iron levels. tional to the acid consumption. Thus, by employing an APU Since the APU recovers free (ie, unused) acid in solution, system, neutralization chemical costs can also be reduced by higher sulfuric acid concentrations can be used economically a factor of approximately one-third. on a continuous basis. This can lead to significantly in- Lime neutralization generates large amounts of calcium creased productivity levels. sulfate (gypsum) sludge in addition to iron hydroxide. Neu- ~~ A plant using a batch sulfuric acid pickle typically formu- tralization with caustic soda reduces solid waste but is usual- lates a fresh bath with 15%w/w (165 ghitre) fresh acid. Spent ly prohibitively expensive. The amount of sludge generated pickle liquor is often mixed with the virgin acid to provide by lime neutralization with and without an APU, assuming some iron (1to 2%) in solution, limiting the aggressiveness of the dewatered sludge contains 70% water, can be calculated the acid. Spent baths usually contain 8 to 10%free acid and 7 as 0.0483 and 0.0725 tons acidlton metal pickled, respective- to 8% dissolved iron (typically [HzS04] = 110 ghitre, [Fe] = ly. 80 ghitre). Iron hydroxide, Fe(OH)2, with or without APU is 0.0268 These bath compositions can be related to minimum pick- tons/ton metal pickled. ling time (Fig. 4).3 The heavy line shown in Fig. 4 represents Thus, the amount of iron theoretically dissolved and the the operating line of the pickle bath. It shows the pickling resulting sludge is the same regardless of whether the system time to vary between 56 and 103 s. The average time (point is employed. In practice, however, the better control that the A) is 80 s. The operator would have to continually adjust process affords helps eliminate over-pickling, tending to re- pickle times to avoid either under or over-pickling. duce the amount of iron hydroxide produced as well as the An APU system can continuously maintain a pickle bath amount of acid consumed. at 15%free acid and 4 to 6% dissolved iron. Pickling times Total sludge without APU is 0.0725 + 0.0268 = 0.0993 within this range (point B) are 62 to 64 s. This represents an tons/ton metal pickled and with APU = 0.0483 + 0.0268 = average improvement of 16 to 18 s or approximately 20%. 0.0751 tons/ton metal pickled. Thus, for example, a 6-day work week could be shortened to The amount of solid waste generated is, therefore, reduced five days. If 20% free acid is used, pickling times can be re- by 24%. duced to 52 s. In this case, a 24-hr/day operation could elimi- In summary, the use of an APU on a sulfuric acid pickling nate one shift per day. line can accomplish the following: In addition to improving productivity, the system will also Elimination of downtime to dump and replace spent stabilize the pickling operation. Pickling rates are constant pickle liquor. when the acid and iron levels are controlled which makes it Elimination of shock loading on the waste treatment easier for line operators to insure that uniform, high-quality system that occurs when a spent bath is dumped. material is produced. Elimination of clogged pipes and pumps with ferrous Acid savings - If the amount of metal dissolved is 0.5% of sulfate crystals. the material processed and the loss of pickling solution to 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%. Fig. 4 - Pickling times in sulfuric acid.3 120,

I_ Hydrochloric acid pickling Many pickling operations have converted from sulfuric acid 100 to hydrochloric acid because of the increased speed and su- __ i" perior surface finish produced. Despite the advantages of -m HCl, others have not converted because HCI regeneration systems were too expensive and complex and fume control f 80 can be problematic. The availability of the APU now makes K 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 lit- tle 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. 0 5 10 15 20 25 (1 8) (3 7) (5 5) (7 4) (9 21 Fume reduction - One of the major disadvantages of HCl Concentration Of Ferrous Sulfate, YO is its strong tendency to fume. The amount of fuming in- (Fe %) creases rapidly with HC1 concentration and temperature.

January 1990 Iron and Steel Engineer 57 Although not widely recognized, the concentration of iron 24 min. Weekly, the contents of tank No. 1are pumped out chloride in the solution also greatly affects fuming. and hauled off-site by tanker. Tank No. 1 is cleaned of Equation (1) relates the concentration of HCl fumes over a sludge and the contents of tank No. 2 are transferred to tank pickle tank to acid concentration, iron concentration and No. 1. Tank No. 2 is then made up with fresh HCl and city temperat~re.~ water. The downtime for this process is approximately 4 to 6 hr. Use of an APU would eliminate this labor and downtime log [HCl] (gas) = -4.369 + 0.030877' + 0.2028[HCl] (solution) by extending bath life indefinitely. + 0.07313[FeClz] (solution) (1) Field tests at the same installation have shown that a satisfactory pickle can be obtained in a single-stage, 23-min where [HCl] (gas) = fume concentration, g/cu metre of inert pickle with a bath containing HC1= 222 g/litre and [Fe] = 35 gas. [FeClz] (solution), [HCl] (solution) = concentration in gflitre. An APU makes such a bath composition feasible pickle solution, weight %. T = temperature of pickle tank, without paying a penalty for waste disposal. This would re- "C. sult in the elimination of one pickle tank, a 100%increase in ~~ The following empirical equation has been developed to productivity, plus an additional 4 hr of productive time estimate the descaling time required for hot-rolled low which is presently lost when the tanks are dumped and carbon steel.5 cleaned. 1569 Log t = -1.356 -0.867 log [HCl] + Energy savings - The trend in continuous HCl pickling (T + 273) has been to increase temperatures to allow higher pickling where rates at lower acid concentrations. With an APU, it is possi- t = descaling time, s ble to utilize relatively high acid concentrations to obtain [HCl] = concentration of hydrochloric acid, gflitre satisfactory rates at lower temperatures. For example, to T = temperature of pickle tank, "C achieve a 30-s pickle at [HCl] = 45 gflitre and [Fe] = 100 g/ litre the temperature must be 95°C (203"F), while the tem- Assuming that a satisfactory pickle of steel strip in 30 s is perature can be reduced to 63.5"C (146°F) if the bath is desired, with a bath containing [HCl] = 45 gflitre and [Fe] = maintained at [HCl] = 130 gflitre and [Fe] = 50 gflitre. 100 gflitre [HCl] = 3.74% and [FeCl] = 18.9%),a temperature The main energy loss from a pickle tank is due to evapora- of 95°C can be calculated from equation (2). Under these tion of water from the surface of the solution. Based on data conditions, a fume concentration of 5.06 g/cu metre can then presented by Rausch: evaporation from a pickling bath can be determined from equation (1). Assuming the solution is be expressed by the following equation when the air velocity discharged continuously to maintain this composition, a loss across the top of the tank is 0.5 metre/s: of 0.45 g of HCl/gram of dissolved iron in the liquid waste would result. log E = -0.7841 + 0.025503 T (3) Through use of an appropriately sized APU, it is possible where to operate the pickle tank at [HCl] = 100 gflitre and [Fe] = E = evaporative rate, litres/hr/sq metre 50 gflitre ([HCl] = 8.82%, [FeC12] = 10%)while losing approximately the same quantity of acid in the waste. At this T = solution temperature, "C composition, the bath operating temperature could be re- The evaporation rates at 95°C and 63.5"C are calculated as duced to 70°C and still achieve a 30-s pickle. The resulting 43.5 and 6.84 litres/hr/sq metre, respectively. The reduction fume concentration would be reduced to 2.05 g/cu metre, a in temperature would then result in an 84% reduction in 60% reduction. evaporative losses with a corresponding reduction in energy requirements for heating the pickle bath. Productivity improvements - If fume reduction is not a Fume losses (4.092 g/cu metre) would be approximately priority, productivity can be improved without adversely increasing the waste problem. By operating the bath at [HCl] 19%lower by operating with the APU at the lower bath temperature, despite the higher acid concentration. = 100 gflitre and [Fe] = 50 gflitre ([HCl] = 8.82%, [FeC12] = 10%) and maintaining a temperature of 95"C, the pickling The various options for operating an HC1 pickling line with and without an APU are summarized in Table 11. De- time can be reduced to 15 s, a 100%increase in productivity. While the fuming losses would increase (12.1 g/cu metre), pending on the individual circumstances, a unit can often be justified on continuous hydrochloric acid pickling lines on the acid losses in the waste would be the same as if no APU were utilized. the basis of the following potential benefits: Many HC1 pickling operations operate a series of two or Elimination of downtime to dump and replace spent more countercurrent pickle tanks. The lead tank is dumped pickle liquor. periodically and the content of each of the other tanks is A 60% reduction in HCl fumes or a 100% increase in transferred to the next tank. This procedure is effective in line productivity or an 84% reduction in energy con- maximizing acid utilization and minimizing waste. However, sumption. it is labor intensive and results in an appreciable amount of nonproductive time. For example, one batch wire pickler operates two HC1 Stainless steel pickling pickle tanks. Wire bundles are placed in tank No. 1 for 24 Mixtures of nitric and hydrofluoric acids are used to remove min, then removed and placed in tank No. 2 for an additional scale from most grades of stainless steel. During the pickling -

TABLE 111 HCI pickling conditions Bath composition Plckiing Temperature, HCI, Fe lime, Fumes, Evaporation, System Requirement "C g/iitre g/litre S g/cu metre litres/hr/cu metre

No APU - 95 45 100 30 5.06 43.5 With APU Faster pickle 95 100 50 15 12.10 43.5 With APU Less fumes 70 100 50 30 2.05 10.0 With APU Less energy 63.5 130 50 30 4.09 6.8

58 Iron and Steel Engineer January 1990 process, iron, chromium and nickel are dissolved in the acid. the optimum pickling bath operating parameters were de- Generally, these baths are formulated with 10 to 15% by termined and adopted. Based on concerns for a healthier weight nitric and 1 to 4 % hydrofluoric acid. Bath tempera- working environment and improved bath heater operation tures are maintained at 55 to 65°C. and maintenance requirements, bath temperature was set at APU resins are susceptible to strong oxidation in hot ni- approximately 28°C. Typical results are shown in Table 11. tric acid. For this reason, the pickle liquor must be cooled to Although analysis of chromium and nickel are not shown, less than 32°C (90°F) before treatment. PTFE-filled graph- other tests have confirmed that these metals are removed in ite or PVDF heat exchangers are employed for this purpose. proportion to the iron. Concentrated nitric and hydrofluoric acid must be added These data show that the APU is recovering 79% of the regularly to maintain free acid levels. However, when the free hydrofluoric acid and 95% of the free nitric acid, while metals content of the solution exceeds 5 to 6%, metal salts removing 89% of the metal contamination. begin to crystallize out of solution and the bath must be The present average pickling time, with the unit installed, dumped. For example, one particular continuous stainless is consistently close to 55 min and it is no longer necessary to steel strip annealing pickling line is shut down for several increase the bath temperature to compensate for reduced hours once every 10 to 15 days to dump one 29,000-litre pickling rates. Acid losses due to fuming and the associated (7700 gal) pickle tank. Approximately 5 cu metres of sludge health and safety hazards have been eliminated. must be manually shoveled from the tank each time. This Generation of sludge in the pickle tank has been dramati- task is extremely unpleasant and hazardous for plant per- cally reduced, thereby eliminating periodic tank dumps with sonn e1. the associated production downtime and labor require- By utilizing an APU, metals can be continuously removed ments. from the pickle tank, so that precipitation of metal salts does Pickling quality is consistently better. Recommended an- not occur. The filtration system in the unit continuously re- alytical methods were implemented. As a direct consequence moves inert solids from the pickle liquor and, by making reg- of the reduced average pickling time and improved surface ular acid additions to the tank, the life of the pickle solution finish consistency, the pickling line production rate im- can be extended indefinitely. Production downtime to dump proved by 60% on a total length treated basis, or by 45% of a the bath and clean the tank, is eliminated along with the total weight basis. Plant management believes that there is associated labor. potential to reduce the pickling times further, to approxi- Pickling rates vary linearly with the free HF concentra- mately 45 min, thereby further increasing the pickling pro- tion in the pickle liq~or.~With an APU it is possible to main- duction rate. tain a higher free HF concentration in the bath (to decrease Total acid consumption was reduced by 71% during the pickle times), without significantly increasing the consump- first five months of operation, despite the fact that the faster tion of hydrofluoric acid. pickling times resulted in 60% more production. In addition, this evaluation was done during the initial period during Case study - A case study recently performed at a batch which familiarization with the system and the new analytical stainless steel tube pickling operation in Markham, Ont., il- techniques were taking place. It is expected that the 1989 lustrates the benefits derived.s figures will show further significant improvements. Prior to installation of a unit, it was common practice to dump a 20,000-litre (5280-gal) pickling tank and recharge Pollution abatement - Nitrate and fluoride ions are dif- the tank with fresh acid feed at 12 to 14-week intervals. ficult to treat and are considered pollutants. With environ- Throughout these intervals, pickling times would increase as mental laws becoming increasingly strict, the reduction in the active concentration of hydrofluoric acid decreased with nitrate and fluoride levels in the final effluent is also an im- increased dissolved metals. Additional hydrofluoric acid was portant benefit. added occasionally when the average pickling time exceeded Reduction in nitrate and fluoride levels are commensurate 2 hr. Some nitric acid was also added to top up the level of with the reduction in purchases of the acids. the bath. An APU installed in a stainless steel pickling line can offer In an effort to reduce the pickling times, the bath tem- the following benefits: perature would sometimes be raised to 40 to 50°C. Unfortu- nately, the increased bath temperature resulted in increased Consistent pickle liquor composition and pickling evaporative acid losses and objectionable, unsafe working performance. conditions around the pickling tank. Generally, the average Decreased pickle times and increased productivity. pickling time per charge was approximately 84 min over the Elimination of downtime to dump and replace spent life of the bath. pickle liquor. Incidents of under and over-pickling were sometimes en- Elimination of labor for removal of scale and crystals countered. These were due not only to the decreasing acid on the bottom of the pickle tank. activities over the life of the bath, but also because reliable Elimination of shock loading on the waste treatment acid and iron analyses were not available to determine when system that occurs when a spent bath is dumped. the bath needed replenishment or dumping. (Proper bath Reduced purchases of nitric and hydrofluoric acids monitoring techniques should be considered as part of the and resulting effluent levels. application of any acid purification system.) A considerable Reduced operating temperatures, fuming and energy amount of time was, therefore, invested in developing new consumption. techniques and refining established techniques for bath analysis. Summary The dissolved iron level would rise typically to above 45 g/ litre by the time the bath was dumped. And, as discussed An acid recovery system based on the APU can offer sub- previously, significant ferric fluoride sludges tend to form in stantial benefits to pickling operations through the pickling baths at such high dissolved iron concentra- Pollution abatement. tions. In fact, extensive cleaning of the tank was usually Resource recovery. needed at dump time to remove the accumulated sludge Energy conservation. from the tank bottom. These sludges might also act to re- Increased productivity. duce the hydrofluoric acid bath activity over long time inter- Reduced line size. vals. In Nov. 1988, a few months after the APU was installed, The simplicity, small size and low cost of the unit make

January 1990 Iron and Steel Engineer 59 recovery feasible, even in cases where conventional regener- 4. Hudson, R. M., and Warning, C. J., “Minimizing Fuming during ation systems have been previously considered and rejected. Pickling with Hydrochloric Acid,” Sheet Metal Industries, June 1969. 5. Hudson, R. M., and Warning, C. J., “Factors Influencing the REFERENCES Pickling Rate of Hot-Rolled Low-Carbon Steel in Sulfuric and Hydrochloric Acid,” Metal Finishing, June 1980. 1. Brown, C. J., “Acidmetal Recovery by Recoflo Sorption,” Roc. 6. Rausch, W., “Die Phosphatierung Von Metallen,” Eugen Lenze 23rd Conference of Metallurgists of the CIM,Quebec City, Can- Verlag, Saulgau, Wurtenberg, BRD, p 233. ada, Aug. 1984. 7. Covino, Jr., B. S., “Fundamentals of Stainless Steel Acid Pickling 2. Brown, C. J., “Treatment of Plating Wastes,” Metals Handbook, Processes,” U.S. Bureau of Mines Information Circular 9195, Ninth Ed., Vol. 5, Surface Cleaning, Finishing and Coating, 1988. American Society for Metals, pp 310-319. 8. “Eco-Tec Nitric/Hydrofluoric Acid Purification Units Saves - 3. ASM Committee on Pickling of Iron and Steel, “Pickling of Iron Money and Permits Simpler, More Consistent Pickling of Stain- and Steel,” Metals Handbook, Ninth Ed., Vol. 5, Surface Clean- less Steel Products,” Eco-Tec Inc., Engineering Bulletin No. 89- ing, Finishing and Coating, American Society for Metals, p 74. 1, Jan. 1989. A

60 Iron and Steel Engineer Printed in USA. 1000/2/90