Jay B. Dwyer. Senior Systems Engineer

Transcription

1 CAUSTIC ETCH REGENERATION U.S. FILTER, INC. - WARRENDALE, PENNSYLVANIA Edward P. Klein Manager, Aerospace / Anodizing Systems Jay B. Dwyer Senior Systems Engineer SUMMARY.') Interest in caustic Etch Regeneration (CER) has risen considerably during the past few years. Increasing pressure from the EPA to minimize waste generation, restrict solid waste disposal practices and improve effluent discharge quality (especially reducing salt concentrations) have been major factors. CER provides the opportunity to make substantial reductions in waste generated from chemical milling and etching processes and at the same time, offars a clear bottom line improvement to operations. The reduction in etchant usage cost provides a payback, and present sludgr hauling costs can be reduced, and in some cases eliminated, while a return for the sale of 85% dry aluminum trihydrate cake is realized. Many facilities are able to show improvements sufficient to provide complete payback in two years or less. The process was developed by Alcoa during the late '70s and is protected under U.S. Patent No. 4,136,026 issued in January,

2 INTRODUCTION During the late '70s, the Alcoa Laboratories developed a process to regenerate caustic soda and retrieve the etched aluminum as a useful by-product. This process was patented in 1979 as the Alcoa Caustic Etch Regeneration System. At that time, five systems were installed in different facilities around the country. The early systems were installed at a time when caustic was less expensive and the environmental pressures were not so severe. Today, however, the price of caustic is up, as is the value of the trihydrate by-product. Additionally, costs for solid waste landfilling, sewer discharge surcharge and the hauling of wastes have skyrocketed over the past few years. Many sanitation districts around the country have imposed regulations that make it impossible to operate without sophisticated treatment systems or CER. CERwill significantly minimize the waste generated in an anodizing facility. The system can regenerate percent of the sodium hydroxide normally used and reduce the volume of sludge generated in the same proportion. The by-product from the system is a dry, crystalline aluminum trihydrate. A more conventional treatment system produces an amorphous aluminum trihydrate which is difficult to wash and clean and usually contains less than 20 percent solids. The product from regeneration can be easily washed and dried and is discharged from a rotary drum-type vacuum filter at approximately 85 percent dry solids and at a ph of between 8.5 and 9.5. Another advantage of the regeneration system is that it can also reduce the Total Dissolved Solids (TDS) in industrial waste streams. The majority of the dissolved solids in an anodizing plant's effluent are Na,SO, (sodium sulfate), which is formed by the combination of caustic soda and sulfuric acid. The reduction 2

3 in caustic soda use, resulting from application of the regeneration system, can reduce the dissolved sulfate by a factor Of three. The regeneration process will further reduce the size of a treatment system by a factor of four or five and in some cases, may eliminate the need for further treatment. During this past year, Alcoa Separations has started up six new systems. One is an aerospace chemical milling facility while the other five are operating on architectural anodizing etch baths. There are two systems at International Extrusions at Alhambra, California and at Waxahatchie, Texas: two more at Tifton Aluminum in Tifton, Georgia and Delhi, Louisiana: and one at Alum-A-Coat in South El Monte, California. The chemical milling facility is at Douglas Aircraft Company in Long Beach, California. 3 THEORY During the etching process, aluminum readily dissolves in the caustic soda bath, forming sodium aluminate and hydrogen gas: 2A1 + 2NaOH + 2H,O -----> 2NaA10, + 3HZ The reaction is strongly exothermic and proceeds more vigorously as the temperature and/or free caustic concentration is increased. z 2NaA102 Through continued use, the etch solution will become increasingly saturated with aluminum and, unless replenished, it will become steadily depleted in free NaOH. Eventually, the solution will reach a degree of supersaturation in aluminum where aluminum hydroxide will precipitate spontaneously and release sodium hydroxide according to the reaction: + 4H,O > 2Al(OH), 1 + 2NaOH 3

4 Since the NaOH complexed during the etching reaction is liberated during the subsequent hydrolysis reaction, the overall reaction is between aluminum and water yielding aluminum hydroxide and hydrogen. 2A1 +6H > 2Al(OH)3 + 3H2 These reactions are the basis for the Caustic Etch Regeneration process shown in Figure 1. The aluminum dissolution reaction occurs in the etch tank with the ratio of aluminum to total sodium hydroxide at approximately At this level, the solution is supersaturated yet stable enough to resist spontaneous precipitation of aluminum hydroxide within the etch tank. The hydrolysis reaction takes place in the reactor tank where fine particles of crystalline aluminum trihydrate are dispersed throughout the solution. The rate of regeneration is proportional to the seed concentration and is optimal at a slurry loading of about 50 percent by volume. EQUIPMENT AND PROCESS DES CRIPTI ON Figure 2 illustrates the equipment used and flow streams in the CER process. The caustic etch solution is continually pumped to the reactor tank where it combines with make-up water. The makeup water accounts for all evaporation and process losses from both the etching and CER tanks. Additionally, it is used for cake wash on the vacuum filter. 4

5 2NaOH ALUMINUM DISSOLUTION HYDROGEN EVOLUTION t 2AI + 2H20 -> 2NaA102 3H2 2NaA102-4H20 -> 2NaOH * 2AI(OH)3 NET REACTION 2AI + 6H20 -> 3H2 2AI(OH)3 FIGURE 1 CAUSTIC ETCH RFGFNFRATInN DDnf'CCC

6 (FIGURE 2) CLEAR FILTRATE RETURN ) CLEAR F 1 LTRATE ETCH TANK REACTOR MODULE WITH MIXER, AND ROTARY DRUM FILTER WITH CAKE DISCHARGE CHUTE A w s -a&-- CLOUDY FILTRATE RETURN TANK CLOUDY FILTRATE PUMP WATER MAKE-UP AND SPRAY WASH

7 Through continuous agitation, the Sodium aluminate Solution is contacted by the aluminum hydroxide seed particles which induces hydrolysis, forming new particles of Crystalline aluminum hydroxide and liberating free caustic. -3 The rotary drum filter, partially submerged and under internal vacuum, continually picks up slurry fromthe reactor and separates it into its solid and liquid components. The aluminum hydroxide solids are retained by the filter cloth on the drum face, forming a cake which is subsequently discharged. The regenerated etch solution passes through the filter cloth into the drum sections and then through an internal system which distributes it to two filtrate receivers. During the cake formation stage, some aluminum hydroxide solids penetrate the filter cloth. The design of the filter is such that the resultant initial cloudy filtrate is directed to the cloudy receiver. From there, the cloudy filtrate is transferred via the cloudy filtrate pump back to the reactor. Once the filter cake forms and increases in thickness, even the smallest particles cannot penetrate the filter medium (filter cloth plus cake) and the filtrate becomes clear. This clear filtrate is directed to the clear receiver and pumped to a surge tank. J The surge tank controls the variable flow rate of clear filtrate produced by the vacuum drum filter system. The clear filtrate exits the surge tank under constant head and flows via gravity to the etch tank, completing the cycle. Since this return flow rate is less than the input rate from the clear filtrate pump, the surge tank remains full with the excess filtrate overflowing and returning to the reactor. In the event that any solids enter the clear filtrate stream, they will settle in the surge tank. The bottom drain on the surge tank allows transfer of such solids to the reactor. 5

8 On one section of the filter, the vacuum is cut off and about 15 percent of the Al(OH), cake is scraped from the drum face by an adjustable knife blade. As shown in Figure 3, the solids fall onto a chute that transfers them to a collection bin. The Al(OH), byproduct is removed from the system at a rate corresponding to the rate at which aluminum is dissolving in the etching solution. Thus, the aluminum entering and leaving the overall system is balanced and the percent of solids in the reactor remains constant. AS the drum section proceeds past the cake knife, it enters a blowback zone where the filter cake on the remaining 85 percent of the drum's width is discharged via compressed air back into the generator tank. This operation renews the filter cloth and the isolated drum section, now exposed filter cloth, proceeds into the slurry and the cycle repeats giving continual separations. With the Alcoa process, crystalline aluminum trihydrate is continuously removed as an 85 percent solids by-product and the regenerated caustic solution is recycled to the reactor tank. The CER system consists of essentially standard equipment, but each process facility is designed to satisfy the requirements of the particular production operation. To give some perspective of scale, a typical etch recovery operation in an extrusion anodizing line would need approximately 200 square feet of floor space. The reactor would be = 10 ft. in diameter by 8 ft. high with a volume of = 4,000 gallons. The rotary drum vacuum filter would typically be 6 ft. in diameter with 4 ft, face, while the skid-mounted vacuum pump, filtrate receivers, filtrate pumps, integral piping and control panel occupy approximately 40 square feet of floor space. The surge tank is usually = a 300-gallon capacity. The holding tank (used to store regenerator liquor while replacing the drum filter cloth) is usually a 1,000-gallon capacity. 6

9 CONCERNS OF THE ARCHITECTURAJi AN ODIZE R -3 A major concern of the architectural anodizer is the surface finish provided by a regenerated etch bath. Etching is specifically included in the process to provide a uniform, matte finish that will mask all the surface irregularities that might appear on a raw extrusion and provide an appealing finished product. The degree to which etching will provide a uniform matte finish, at a minimum surface removal, is justifiably of concern to the anodizing manager. Laboratory work, and more importantly, field experience, indicate that such a finish can be obtained equally as well in a regenerated etchant as in any other. One important factor that would affect the surface finish is the dissolved concentration in the etch bath. It is important in order to maintain a good, consistent finish to keep this concentration in the supersaturated range. Some anodizers are using additives to assist in this process while others are attaining excellent results without additives. Recognizing the importance of measuring the quality of surface finish, a method was developed using a simple reflectometer to measure the degree of specularity of an anodized surface. This has been a great tool in ensuring excellent surface characteristics using regenerated baths, especially in light of the fact that every sample is different and that results are affected by original surface condition, metallurgy, heat treat condition and the condition of the anodize bath. Another question that has been asked frequently is: How much of the caustic soda can be regenerated and how much reduction in sludge can be realized? 7

10 i With the Alcoa process, crystalline aluminum trihydrate is continuously removed as an 85 percent solids by-product and the regenerated caustic solution is recycled to the etch tank.

11 3 The volume of caustic that can be regenerated is limited only by the amount of caustic that is dragged out of the etch bath with each workload. The process Will, theoretically, regenerate 100 percent of the caustic, as the equation is 100 percent reversible. The dragout represents the only chemical loss from the system. The amount of caustic left in the dry cake after wash is insignificant. It is advisable to operate the etch bath at as low a total sodium hydroxide content as possible so that the amount dragged out with each load is minimized. When the sodium hydroxide is lower, fewer gallons are dragged out with each workload, due to the reduction in viscosity, and, more significantly, less hydroxide is included in each gallon dragged out. Experience indicates that the total caustic usage in an anodize facility can be reduced by 85 percent. The sludge generated in wastewater treatment will only be that from the dragout and, therefore, that also will be reduced by 85 percent. In chemical milling facilities, savings of 95 percent are attained because the dragout is inherently much less. Another concern of the anodizer relates to the chemical balance in his plant. One may ask: If I regenerate the caustic, what good does that do? My plant is presently balanced and the effluent is neutral. Most anodizers operate so as to produce a neutral plant effluent (between ph 6 and 10). Theoretically, the amount of acid required to anodize a given amount of square feet of aluminum is percent of the required caustic. When one regenerates, then the acid usage can also be reduced by percent simply by making adjustments in the process. A portion of the remaining waste acid is neutralized by caustic dragout and it will be necessary to procure fresh caustic, ammonia or lime to neutralize the remaining acid. However, the overall economics weigh heavily in favor of regeneration. In some cases, acid purification can be justified to recover the remaining acid to balance out the process. a

12 ECONOMICS conventional treatment of spent caustic etch involves large, expensive equipment and high operating costs. It also generates voluminous quantities of gelatinous sludge that is difficult to de-water and costly to transport. Solid waste disposal costs, coupled with the cost of the caustic soda, lean heavily in the direction of recovery systems. CER technology represents an attractive solution to these problems. The system will recover 85 percent of the caustic soda and provide a dry crystalline alumina by-product which is readily salable to suppliers of alum. The ATH by-product is used as a feed stock for alum which can be used in the paper products, petroleum and ceramic industry. It is also used in wastewater treatment and for water purification, depending on its purity. Moreover, the process J eliminates costly downtime due to decanting and recharging tanks and provides a uniform quality etch bath with consistent product quality. The bottom line return can pay for the system installation--in many facilities-in two years or less. 9