As environmental and transportation regulations become more stringent around the world, treating spent acids is becoming less of an option and more of a necessity
Acids play many different roles in a large number of sectors of the chemical process industries (CPI). They are used as catalysts or as reagents in chemical synthesis; they are used for leaching metals from ores; for purifying and plating of metals and for cleaning metal surfaces. In some cases, they are also produced as byproducts, and recovered as solutions to prevent their release into the atmosphere as gases.
In the course of any of these processes, the acid becomes diluted or contaminated with impurities, or both, creating a waste stream — often in very large volumes — known as spent acid. Re-processing this waste stream is more and more becoming the responsibility of the producer.
In the past, such spent acids — depending on the contaminants — could be neutralized and discharged into wastewater treatment plants, or sold to other CPI sectors that could use the acids for purposes where the contaminants would not be problematic. Or the spent acids could be transported to toll manufacturers that specialize in treating the spent acids — removing the contaminants and recovering, purifying and re-concentrating the acid for resale.
Today, many of these options are drying up. Tighter environmental regulations are making disposal costs too high. Rules for transporting hazardous waste are making it too expensive to send spent acids elsewhere for reprocessing. Industries that were once able to use the spent acids — such as the fertilizer industry — are disappearing from areas where the spent acids are generated. “In Europe, the fertilizer industry essentially does not exist anymore,” says Keith Bundil, director of Process Innovation, Pfaudler Group (Neu-Isenburg, Germany; www.pfaudler.com). As a result, “operators are left with only two choices: invest in acid recovery, or stop production,” he says. Of course, they can also move production to where there are more disposal options available for spent acids, but even in countries like India and China, environmental regulations and restrictions on transporting hazardous waste are becoming stricter, says Bundil. “Therefore, concentration and regeneration of spent acids is becoming more and more important.”
“Probably the major change in the last few years is the increased focus on acid recovery in Asia (especially China) and India,” agrees Ken Armstrong, director of technology & Development at Chemetics, Inc. (a Jacobs company; Vancouver, B.C., Canada; www. jacobs.com/chemetics). “Acid recovery is almost always based on economics: if it is cheaper to give/sell the spent acid to use for another purpose, then this is preferred. If this is not possible or allowed, then cleaning and re-concentration will be the next cheapest option. If no other alternatives exist, then companies either contract out or build a thermal destruction plant to deal with the acid, such as a sulfuric acid regeneration [SAR] plant [Figure 1]. This is the most expensive solution,” he says. “In Southeast Asia, we have seen more local regulatory pressures on companies to deal internally with their own spent acids and avoid sending it offsite to others to handle,” explains Armstrong. “Essentially, this is just the same general global trend to reduce, reuse and recycle byproducts and wastes within your own facility.”
An expensive necessity
Today, it is still extremely expensive to recover and concentrate acids, like sulfuric acid (up to 96 and even 98 wt.%), with capacities of more than 20 tons per hour, and with removal of inorganic or organic impurities. You need special materials of construction and a lot of energy, says Bundil. That means materials such as glass, glass-lined steel, tantalum and niobium — “never simple stainless steel,” he says. Although the basic chemistry and physics for acid recovery are no different than they were decades ago, there have been constant evolutionary improvements in acid-recovery technology. Being more than two decades in this business, the development of more sophisticated technologies is essential, says Bundil, such as the change from atmospheric to vacuum processes (Figure 2) back in the 1980s and other efforts to reduce energy requirements, such as internal energy recovery and multi-effect evaporators.
Unlike engineering companies that design and build acid-recovery plants, Pfaudler Group provides a more holistic strategy, offering the benefit of single source responsibility for acid-recovery plants and systems, starting with the design, engineering and supply of the relevant equipment, followed by construction management, commissioning and maintenance assistance all the way until the end of the lifecycle of such a plant, says Bundil. This “one-stop” strategy includes the complete range of services and supplies, necessary for a successful implementation of an acid-recovery plant, he explains.
So, in addition to the core business of glass-lined (Glasteel) and alloy equipment, the Pfaudler Group offers several branded product lines that include Edlon, fluoropolymer-lined column and column internals; Normag, borosilicate 3.3 glass equipment and systems; Montz, engineered column systems and column internals; Mavag, specialized in drying and filtration applications; and Interseal, which includes a patented dry-run sealing technology for rotating equipment.
Each application is different
Although there are just a few important mineral acids — HCl, HF, H2SO4, HNO3 — the number of different spent acids requiring treatment is much higher. Depending on the process, the spent acids are generated in different concentrations, or as mixtures of several acids, or with different contaminants. “Mixed acids are not the problem; instead, it is the accompanying impurities that make each application different,” says Edgar Steffin, head of marketing, De Dietrich Process Systems GmbH (Mainz, Germany; www.dedietrich.com). Common acid mixtures are effluents from nitration processes used for making toluene diisocyanate (TDI), nitro cellulose or explosives, which contain H2SO4, HNO3 (Figure 3) and organic components, he explains. For a long time, it has been possible to separate the acids from H2SO4/HNO3 mixtures. Today, the challenge is to do this using as little energy as possible, and to recover or get rid of the organic components, says Steffin.
Most spent-acids-concentration plants contain some novelty to handle the impurities in the acid, concurs Chemetics’ Armstrong. “The acid-concentration part is never the challenging aspect of the project. Dealing with the impurities in the spent acid is always a challenge, and can be the difference between an easy, cheap solution and an expensive one,” he says. For example, a major process Chemetics has developed in recent years is one to purify and recycle methyl chloride spent acids. “This process was custom developed for a client and is now in operation in three locations,” says Armstrong.
In the production of methyl chloride, concentrated H2SO4 is used to remove impurities — mainly di-methyl ether, which is formed as a byproduct, and unreacted methanol — from the methyl chloride vapor stream exiting the reactor. The spent acid leaving the plant also has small amounts of methyl chloride.
The success of the recycling of this waste stream depends on the effective removal of the contaminants, says Armstrong. Methanol reacts with H2SO4 to form methyl sulfuric acid, which is difficult to remove from spent acid, so the first step is to “liberate” the methanol and recover the H2SO4 in the hydrolysis unit. The second step then removes the organic contaminants from the acid, which eliminates problems in the downstream acid concentration. The process is said to offer “virtually complete” removal of contaminants — more than 99% recovery of sulfuric acid for recycle to the methyl chloride plant, and more than 70% recovery of methanol for recycle, according to the company.
Mixed acid recovery has been the biggest change in applied acid recovery technology, says Bryan Cullivan, president of Beta Control Systems, Inc. (Beaverton, Ore.; www.betacontrol.com). Whether HNO3 /HF or HCl/HF, the cost of the acid and the cost of disposal have opened a marketplace for acid recovery that the cheaper HCl and H2SO4 markets did not enjoy, he says. “We recently introduced the Mixed Acid Recovery System into the aerospace and stainless steel markets to compete with acid retardation. The capital return is much quicker for the more expensive acids,” he says. “Using vacuum evaporation coupled with a forced circulation co-flash approach, we were able to provide excellent recovery of HF, HNO3 and HCl in a relatively compact design. As a result of the extreme corrosiveness of the acids, all of the components in the process had to withstand both the temperature and the aggressive environment to be practical. The patent process is underway and so far, the first applications continue to perform,” he says.
In the mixed-acid recovery process (Figure 5), a waste acid is pumped through a pre-filter and into the evaporator loop, comprised of an evaporator exchanger and a separator tank. In the evaporator loop, spent acid is pumped under slight pressure through a corrosion-proof heat exchanger. The spent acid solution is heated past the boiling temperature of both acid and water. “Once the solution is released into a controlled vacuum environment, both the mixed acid and water vaporize in our unique ‘co-flash’ reactor,” he explains. The acid and water vapors travel upward through the liquid/vapor separator tank and into the rectifier. The remaining metal salt solution (metals + H2O) continues to circulate through the pressurized boiling loop until it reaches a specific concentration and is discharged to a storage tank.
In the rectifier, the concentration of acid is controlled to return excellent quality, re-concentrated acid to the process tank. The remaining water vapor, stripped of HNO3 and HF, continues into the condenser where it is sub-cooled and condensed to good quality water. This water is typically reused in the process.
One of the company’s most recent projects has been the recovery of HF and HNO3 from titanium milling. “The separated product was not only the high-valued acid, but also the recycled titanium,” says Cullivan. He also points out that Beta installed the largest sulfuric acid recovery system in North America two years ago. The system is essentially two systems with a common walkway and has a processing capacity of 32,000 gal/d of spent acid.
Whereas large-scale acid recovery plants continue to be thermal-based processes, smaller systems based on acid dialysis have been developed by Mech-Chem Associates, Inc. (Norfolk, Mass.; www.mech-chem.com). These compact modular units are ideally suited for recycling sulfuric acid used in aluminum anodizers and the metal plating industries, says marketing manager Ian Butler. “These systems have demonstrated improved anodize quality, producing consistent anodize color and consistent anodize thickness, while reducing bath dumping and makeup,” he says. The modular, compact units (Figure 6) can be adapted to handle the production volume requirements of a given application, with commercial modules handling capacities of 15 to 1,000 gal/d. Larger capacities are realized by adding membrane modules in parallel. A laboratory unit is also available for evaluation purposes, and the data gathered are used for scaling up to pilot or full-scale installation at a facility.
The systems are based on a membrane-separation process known as diffusion dialysis. Two feed streams flow countercurrent between an anion-exchange membrane, which acts like a semipermeable barrier between the water stream and the acid stream (which contains the dissolved metals). Driven by a concentration gradient, the acid permeates through the membrane into the water stream, while the metal ions are rejected by the membrane, thereby creating a purified sulfuric acid solution. The system can remove and control other contaminant build-ups in the anodized bath, such as copper, iron, lead, magnesium, manganese, phosphate, silicon and zinc, while producing a minimum of rejected waste byproduct for subsequent treatment and disposal, says Butler. Typically, 80–95% of the acid is recovered with 80–95% of the metals removed, on a single pass-through, according to the company.
In addition to dialysis technology, Mech-Chem provides large-scale thermal-separation technology for acid recovery. For example, the company is currently involved in acid purification, using distillation/fractionations, of electronic-grade HF, HNO3, and H2SO4, says Butler.
Regenerating HCl from pickling
Hydrochloric acid is commonly used in the pickling lines of steelmaking, a cleaning process that generates large volumes of acidic rinse water, iron chlorides, metallic salts and waste acid. For such large-scale operations, it may make more sense to regenerate the acid from the iron chlorides, rather than recovering the spent acid. For the HCl regeneration, two main processes are used: fluidized-bed (FB) and spray roaster technologies, says Herbert Klausner, senior technologist at Tenova Key Technologies Industriebau GmbH (Vienna, Austria; www.tenova.com). Although the company offers both, the market is moving more towards the spray roaster technology, mainly driven by economical aspects, he says. Spray roasters operate at a temperature of around 640°C, compared to 950°C for FB, for example.
In the process, waste acid is preconcentrated, then pumped to the roaster, where it is injected. The liquids are circulated in a venturi, and the ferric and ferrous chlorides are roasted into iron oxide pellets, with the release of HCl. The solids are recovered at the bottom, and the HCl gas recoverd in an absorber column as nealry azeotropic hydrochloric acid (about 18 % by weight). The regenerated acid can then be stored and reused in the pickling plant.
In recent years, Tenova has improved its Spray Roaster technology, with a significant enhancement of operating costs, product quality and environmental protection. Under the BLUEdriven trademark, the most recent innovation has been the BLUEdriven Flex Capacity, which makes it possible to cover a very wide plant capacity. If the capacity of the pickling plant is increased, the regeneration capacity can be added in parallel to the existing acid-regeneration plant in just a week or two, without exchanging equipment. This enables small-scale or startup companies to have a mid-term investment schedule, with the ability to increase capacity to 100% at a later time. Also, the metal-oxide pellets that are formed from the pickling sludge can be fed directly into the blast furnace, without the need for sintering (a process that operates at 1,300°C). The hardening process does not require an oven, says Klausner.
The first plant with BLUEdriven Flex Capacity started operating this summer, and a second plant with the same characteristics will start up at the end of 2018.
In addition to its activities in the steelmaking industry, Tenova is also seeing interest in acid recovery for the titanium and magnesium industries. The company is also working on new applications for regenerating organic acids from the production of biodegradable plastics. This development could also be used in many areas of the food industries, Klausner says. The process is being developed in a pilot plant.
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