Environmental II Session F Acid Purification and Recovery Using Resin Sorption Technology-A Review Mike Dejak and Kevin Munns Eco-Tec Limited 925 Brock Road, South Pickering, Ontario L1 W 2x9
Acid Purification and Recovery using Resin Sorption Technology - A Review by: Mike kjak and Kevin Munns ECO-TEC LIMITED 925 Brock Road, South Pickering, Ontario L1W 2x9
In the metal finishing industries solutions of strong mineral acids (sulfuric, nitric, hydrochloric) are used to remove surface oxides that impair subsequent manufacturing operations. To a large degree these oxides are dissolved by the acids and as the concentration of dissolved metals increases there is a corresponding decrease in free acid concentrations. Although fresh acid can be added to the solution to make up for this, the oxide removal (pickling) rate generally decreasds when the metals concentrat ion increases. The bath must be dumped when the oxide scale is no longer being adequately removed. This method of operation can cause productivity related problems. Freshly formulated pickling baths containing very low levels of dissolved iron are very aggressive and can cause overpickling. Material of substandard quality can also be produced as the bath approaches the spent condition. Rework may be necessary to prevent problems with subsequent surface treatment processes. In addition, spent pickle liquors can contain large amounts of unused or "free" acid. Free acid in a spent pickle bath represents a loss of valuable chemicals and a waste treatment liability. Continuous purification of pickle liquors can, therefore, offer several benefits to steelmakers: - uniform product quality with reduced rework - increased average pickling speed - reduced chemical consumption - reduced waste handling requirements 2
2.0 The APU Process Certain ion exchange (1x1 resins, similar to those used in water softeners and water deionizers, have the ability to absorb strong acids while excluding metal salts of these acids. The ac.ids can readily be desorbed from the resin beads with a water wash. While this phenomena, known as "acid retardation", was observed by researchers in the early sixties' it was not successfully commercialized at that time. This was likely due to the limitations of conventional IX equipment design. While the acid retardation work was underway, a novel ion exchange3 process was being developed at the University of Toronto. This totally new approach to ion exchange - embodied several unique design features: - Fine mesh resin beads... to increase the surface area per cubic foot and improve reaction kinetics. - Short resin beds.... reducing pressure drop and equipment size - Fixed resin beds... to minimize intermixing and dilution of feed and regenerant phases. - Countercurrenc flows... of feed and regenerant steps to maximize chemical efficiency. In 1975 a program was initiated to apply the process to acid retardation. The program was a success and the first acid retardation system using this technology was installed in 1977 4 for the purification of sulphuric acid anodizing solutions. Since that time over 150 similar systems have been installed. 3
3.0 Basic System Design In many pickling operations, the bath is formulated, worked, adjusted for free acid content as required until the dissolved metals reach a critical level, and dumped. Ah Acid Purification System is normally designed to hold the dissolved metal levels at a?referred operating level, typically 50-60% of normal dump concentration. Free acid concentration can be maintained at whatever level is required to provide the necessary pickling rate. A? Acid Purification System process is shown in Figure 1. There are three basic steps in the System including: - Bath cooling (if required) - Bath filtration - Acid Absorption a) Bath cooling Bath cooling is necessary where oxidizers such as nitric acid or hydrogen peroxide are present. While sulphuric and hydrochloric acids czn be treated at temperatures up to 80 C, the presence of oxidizers necessitates that the feed be cooled to about 32 C. 0 0 City water is normally used for cooling. A variety of cooler designs are available depending on the particular requirements of the system. The water may be used for regeneration of the absorption resin and rinsing on the pickle line. 4
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b) Bath filtration Proper filtration of the pickling acid and the regeneration water is necessary to minimize pressure drop across the absorption res in bed. Pickling acid solution is filtered in a multimedia pressure filter. A backup cartridge filter is also used. The amount of solids to be removed will depend on what alloy has been pickled and how much scale has Seen removed, however it should be noted that the solids load is usually much lower with the purification system in operation. This is because dissolved metal concentrations are kept below their solubility limit and crystallization of metal salts does not take place. A monitor on the filter checks the differential pressure between the inlet and the outlet and indicates when a backwash is required. A combination of water and compressed air is used to backwash and scour the media. The backwash sequence begins with an air blowdown to exhaust the entrained 2ickle liquor to the surge tank. The media is then fluidized with a combination of air and water to remove trapped solids. The waste water from the backwash may contain small amounts of acid and should be treated with rinse waters from the pickle line. Water for regeneration of the absorption resin is filtered in a multi-media filter similar to the one used for the feed acid. With this filter no special air blowdown or scouring is required, 6
c) Acid Absorption Acid absorption takes place in a skid mounted device called an R Acid Purification Unit or APU. The APU consists of a container or "bed" of absorption resin, process valves and controllers. It is the heart of the System. Figure 3 is a photograph of an Acid Purification System for sulphuric acid service. The resin bed is a cylindrical vessel 30-60 cm (12" - 24") in height. Bed diameter is dependent upon the feed flow and typically varies between 15-180 cm (6'' and 72"). The APU works on a very simple two - step cycle. During che first part of the cycle, the upstroke, filtered acid is pumped up through the resin bed and free acid is absorbed by the resin beads. Dissolved metals pass through the unit unhindered. This step continues until the resin bed has been fully loaded with acid. Byproduct solution flowing from the unit will contain dissolved metals and a small amount of dissolved acid. In some cases the byproduct may contain recoverable chemicals, however, in most cases it must be neutralized and clarified prior to discharge. The suitability of the plant's effluent treatment system should be checked to ensure adequate capacity is available in neutralization tanks ana filters. 7
After the resin bed has been loaded with acid, filtered water is pumped down through the bed during the "downstroke" part of the cycle. The water desorbs the acid from the resin at a concentration suitable for recycle back to the pickle tank. This step continues until the bed has been fully regenerated. A t the beginning of each of these steps a void solution can be recycled. In the case of the upstroke step this void is essentially just the water that was left in the bed from the preceeding cycle. The void during the downstroke is feed acid that can either be mixed with the purified acid or returned to the feed tank. The unit's operation is regulated by a flow totalizer that is coupled to a programmable controller. It is worthy to note that in all applications, acid is pumped up through the bed and water is pumped down through the bed. This utilizes the density differences of the two solutions to maintain a stable interface between the acid and water, approaching plug flow through the resin Sea. Solution intermixing and dilution is minimized. -8-
WATER c METALLIC SALT I SPENT ACIO DOWNSTROKE SPENT ACIO WATER I I r 1 -I- f PURlF IED ACID RES1 N BED t t. tit.: I.j................. SPENT ACID FIGURE 3: STEPS IN THE AP P PROCESS - 9-
4.0 Applications in the Steel Industry The three pickling solutions used most frequently in the steel industry are based on sulphuric, hydrochloric or nitric acids. 4.1 Stainless Steel (Mixed Acid) pickling Mixtures of nitric and hydrofluoric acids are used to remove scale from most grades of stainless steel. Generally, these baths are formulated with 10-15 percent by weight nitric acid and 1-4 percent by weight hydrofluoric acid, depending on the alloy to be createa. Bath temperatures are maintained at 55-65 degrees C. During the pickling process iron, chromium ana nickel are dissolved into the acid. Concentrated nitric ana hyarofluoric acid must be added regularly to maintain free acid levels, however, when the metals content of the solution exceeds 5-6% w/w, metal salts begin to crystallize out of solution and the bath must be dumped. Frequently, manual labour must be used to remove the sediment from the tank. With the APU System, the total dissolved metals concentration can be continuously maintained in the range of 2-4% w/w. This enables the solution to be used indefinitely. Typical results using the APU on mixed acid service are given in Table 1.
Table 1: APU Results - Nitric / Hydrofluoric Service Component Feed Product Waste Nitric acid (g/l) 120 114 5 Hydrofluoric acid (g/l) 30 24 5 Iron (g/~) 30 9 ia Chromium + Nickel (g/l) 10 3 6 % Loss 5 20 70 70 Relative Flow 1 1 1.2 The concentrations of nitric and hydrofluoric acid shown above are free acid concentrations. The absorption process has been observed to be more efficient with highly dissociated (strong) acids. As the hydrofluoric acid is a weaker acid than the nitric, the recovery efficiency is lower. The installed cost of the APU System is dependant upon the amount of dissolved metals that must be removed from the pickle liquor and can vary between $75,000 and $500,000. These capital costs can be recovered very quickly due to value of the acids involved. An Economic Evaluation is presented for the APU System on mixed acid service in Table 2. The evaluation is based on the following conditions: Spent bath flowrate: 1500 L/hour (400 USGPH) Spent bath composition: 100 g/l HN03 (9% w/w) 20 g/l HF (1.8% w/w) 40 g/l Fe (3.6% w/w) Plant operation: 6000 hours per year
Table 2: cost Component Economic Evaluation on Mixed Acid Service Annual Cost ($/year) simple Neutralization APU system 70% HN03 $150 per tonne 70% HF $950 per tonne Lime $50 per tonne $192,900 $769,500 $86,700 $13,800 $592,800 $46,500 45% Cake Haulage $40 per tonne Labor $15 per hour Replacement parts $173,500 $144,400 $30,000 $12,000 I Utilities $3,000 Total Operating costs Savings compared to Simple Neutralization $1,222,600 $842,500 $380,100 Estimated Capital Costs $300,000 0.79 are expressed in metric tonnes (2200 lbs). b) Chemical costs based on prevailing rates as reported in Chemical Marketing Reporter (7/87).
There is a lesser reduction in hydrofluoric acid purchases than might be expected from the operating data presented in Table 1. It has been assumed for the sake of the analysis that both iron and chromium metals are tri-valent and present in solution as complex fluoride salts. Despite this, the chemical and treatment cost savings can readily justify the capital ex2ense. The evaluation does not attempt to quantify benefits associated with uniform pickling rates (reduced rework) and composition (prevention of salt crystallization and tank cleaning) although these can be very significant. -. 1.3 -
4.2 Hydrochloric Acid Pickling Hydrochloric acid is commonly used for pickling of mild steel on batch and continuous lines. Baths are made up with a free acid concentration of 10-15 percent by weight and can be operated between 20 and 80 degrees celcius. The baths are dumped when the dissolved iron level reaches 8-10% by weipht. It is possible to reduce free acid levels to less than 5% as the bath reaches a spent condition. S?ent acid can be contract hauled, neutralized with lime or, in some cases, used to produce ferric chloride. Continuous strip lines frequently operate three or four pickle tanks in series with the flow of acid running continuously between tanks. This design allows spent pickle liquor to be cohtinuously withdrawn at very low free acid levels. In order to achieve the high pickling rates required in these lines, elevated temperatures must be used. This can lead to problems with acid vapour losses. with the APU System it is possible to reduce the solution temperature by operating the pickle liquor at higher free acid concentrations. This can be done economically since the APU System recovers virtually all of the free acid. In batch pickling operations the level of free acid in the spent liquor can be substantial. The APU can be used to continuously remove iron from the bath thus eliminating bath dumps. This reduces the loss of free acid and the attendant treatment requirements. In addition, by allowing the economical use of higher free acid concentrations and lower dissolved iron
concentrations, pickling speeds can be increased. The elimination of lost production time due to bath dumping and re-formulation is another benefit of continuous purification. Typical results for the APU System on hydrochloric acid service are given in Table 3. Table 3: APU Results - Hydrochloric Acid Service Component Feed Product Waste Hydrochloric acid (g/l) 15C 153 8.6 Iron (g/l) 50 32 29 8 LOSS 4 40 Relative Flow 1 0.34 0.7 The product is returned from the APU in a slightly concentrated state. Consideration must still be paid to the water balance in the pickle tank, particularly since commercial grade hydrochloric acid is a relatively weak (32% w/w) solution. If steam spargers are used for bath heating, difficulties may be encountered with the dilution caused by the condensed steam. Cut-of-line heating systems using shell and tube heat exchangers are preferred to maximize the acid recovery. An Economic Evaluation is presented for the following conditions: Spent bath flowrate: 1500 L/hour (400 USGPH) Spent bath composition : loo's/l HCL (9%) 70 g/l Fe (6.5%) Plant operation: 6000 hours per year
Table 4: Economic Evaluation on Hydrochloric ~ cid Service cost Component 32% HCL $70 per tonne Lime $50 per tonne 45% Cake Haulage $40 per tonne Labor $15 per hour Annual Cost ($/year) simple Neutralization APU system $371,600 $199,500 $94,700 $50,800 $105,300 $98,100 $30,000 Replacement parts $6,400 Utilities Total Operating costs Savings compared to Simple Neutralization $571,600 $1,600 $386,400 $185,200 Estimated Capital Cost $160,000 Payback Period (years) 0.86 Notes: a) All costs are expressed in 1987 U.S. dollars. All weights are expressed in metric tonnes (2200 lbs). b) Chemical costs based on prevailing rates as reported in the Chemical Marketing Reporter (7/87).
This comparison shows the APU System to be a very cost effective option for treatment of hydrochloric acid pickling solutions. As previously discussed, other productivity related benefits have not been considered. In addition to quality related benefits, the APu System can allow higher free acid concentrations to be used economically. This may permit lower bath temperatures to be used. In addition to energy savings, lower temperatures lead to a reduction in HC1 vapor losses and the attendant treatment requirements. 4.3 Sulphuric Acid Picklinq Sulphuric acid is most commonly used in batch pickling operations although, there are still some continuous sulphuric pickling lines in use. Generally, at least 10% w/w sulphuric acid must be maintained in the pickling acid to ensure ninimum pickling rates. The acid becomes spent when the dissolved iron level reaches 6-8% by weight. At higher levels the iron will crystallize out of solution. Care must be taken not to allow either the free acid level, or the bath temperature, to get too high as this will depress the solubility of the ferrous sulphate. In addition to the control considerations required with sulphuric acid, the spent bath can be difficult to treat. Dosing with lime generates large amounts of calcium sulphate (gypsum) sludge. Use of the APU on sulphuric acid pickling solutions can overcome these disadvantages. Continuous bath purification ensures that dissolved iron concentrations don't exceed solubility levels.
Since the APU recovers free ( or unused ) acid in solution, higher sulphuric acid concentrations can be used economically. This can lead to signifigantly increased productivity levels. Typical results are given below. Table 5: APU Results - Sulphuric Acid Service Compo ne n t Feed Product Waste 3 LOSS Sulphur ic acid (g/l) 150 138 10 8 Iron (g/l) 70 26 40 65 Temperature (OC) 80 76 19 5 Relative Flow 1 1 1.15 It is worth noting that the APU System recovers most of the heat in the feedstock. An Economic Evaluation for the APU System on Sulphuric acid service is presented in Table 5. The comparison is based on the following conditions: Spent bath flowrate: 1500 L/hour (400 USGPH) Spent bath composition : 150 g/l H2S04 (13%) 70 g/l Fe (6.5%) Plant operation: 6000 hours per year The APU 2rocess does not remove the ferrous sulphate in a recoverable form and magnesium hydroxide treatment is recommended. While this results in the generation of a considerable amount of ferrous hydroxide sludge, the filter cake is generally considered to be non-hazardous and can be used as a landfill.
Table 6: cost Component Economic Evaluation for the APU System Annual Cost ($/year) Simple Neutralization APU System 93% H2S04 $82 per tonne Magnesium Hydroxide $215 per tonne 45% Cake Haulage $40 per tonne Labor $15 per hour $216,300 $111,aoo $312,100 $161,400 $90,100 $90,100 $30,000 Replacement parts $7,200 Utilities Total Operating costs Savings compared to Simple Neutralization $618,500 $1,800 $432,300 $216,200 Estimated Capital Costs $200,000 Payback Period (years) 0.93 Notes: a) A l l costs are expressed in 1987 U.S. dollars. A l l weights are expressed in metric tonnes (2200 lbs). b) Chemical costs are based on prevailing rates as reported in the Chemical Marketing Reporter (7/87). - 19-
5.0 Summary A proven low-cost purification system based on absorption resin technology is now available for a variety of pickling acids including the nitric / hydrofluoric mixtures used for stainless steels. In addition to reducing acid purchases and disposal requirements, the continuous system allows pickling baths to be economically operated at higher free acid and lower dissolved metal levels than would normally be considered practical. This can lead to significant increases in productivity. 6.O References 1. Metals Ilandbook, 9th Edition, Volume 5, pp 73-74, American Society for Metals, Metals Park, Ohio 2. M.J. Hatch, J.A. Dillon, Industrial and Engineering Process Design and Development, 2, 4, 253 (1963) 3. I.H. Spinner, R.F. Hunter, Canadian Journal of Chemical Engineering, 42, 28, (1964) 4. C.J. Brown, D. Davy, P.J. Simmons, Plating and Surface Finishing, 66, 54 (January 1979) -2c-