Nitric HFl Case Study
Recycling of Nitric-Hydrofluric Acid With Diffusion Dialysis
What is Diffusion Dialysis?
Diffusion Dialysis is a membrane separation process. It has been successfully used for many years for the separation and recovery of acids from dissolved metal bearing solutions. Diffusion is the spontaneous movement of a material from an area of high concentration to an area of lower concentration. Driven by the concentration difference, the movement of material will continue on its own until the concentration difference no longer exists. Dialysis is the separation of molecules due to the differences in the rate of movement of the molecules through a semipermeable barrier.
In the recovery of acids with Diffusion Dialysis, an anion exchange membrane acts as a semipermeable barrier placed between a flowing water stream and a flowing acid solution with dissolved metal. The anion exchange membrane has fixed positive charges located on its surface. These positive charge locations attract the negatively charged anions in solution that come in close contact with the anion exchange membrane surface.
In the case of nitric-hydrofluoric acid pickling solution, the overwhelmingly predominant anions are the nitrate ion, NO3 -1 and the fluoride ion, F -1. As these nitrate and fluoride anions in the acid solution come in contact and are attracted to the positively charged membrane, they diffuse across the membrane into the less concentrated water solution on the other side of the membrane. This is due to the concentration difference across the membrane. Simultaneously, the thermodynamic Law of Electroneutrality (in solution total charge must balance to zero) requires that the transference of every nitrate or fluoride anion across the membrane be accompanied by the transference of a positive charge. Positively charged ions such as the ferrous cation, Fe+2, or the nickel cation, Ni+2 or other metal cations, are strongly inhibited from crossing the positively charged membrane because of the repulsion between like charges. The hydrogen ion, present in the acid solution as H3O+ ions, or protonated water, is also positively charged, but is able to cross the membrane with very little hindrance. This occurs for two reasons: the highly associated nature of water allows the hydrogen ion to effectively delocalize its charge, and because of the high concentration of hydrogen ions in the acid solution.
Applied Diffusion Dialysis
The standard processing rate for Diffusion Dialysis systems is: a liter per hour per square meter(approximately 0.025 gallons/hour/square foot) of available anion exchange membrane surface area. To obtain the necessary membrane area that is required to process large volumes, the membranes are stacked between gasketed hydraulic flow spacers. These membrane stacks are usually standardized over a range of differing processing capacities.
Figure #2 depicts a typical, automatically operated acid recycling configuration. The acid recycling system has two liquid chambers at the top of the unit: one chamber is for water, and the other is for the acid to be processed. A dual set of level controls is located in each chamber. As the acid level drops in the chamber, the primary level controller will energize a self-priming air diaphragm pump located on the system. Acid solution will be drawn into this pump and then sent through a filter and into the acid holding chamber on top of the module.
Once the acid holding chamber has been refilled, the primary level controller will shut off the pump. Should the primary level controller fail for some reason, a secondary level controller will shut off power to the system at emergency-high, or emergency-low level, and an audible alarm will sound.
A similar dual arrangement is present in the water holding chamber. Instead of a pump, the primary level controller is tied into a solenoid valve which is plumbed to the water feed line.
Once the water and acid solutions are in the holding chambers on the unit, they flow independently, by gravity, into the membrane stack(s) on the base of the unit (see Figure #2). The acid and water solutions flow counter-currently through the membrane stack, thus maximizing usage of the concentration gradients. Using the principles of Diffusion Dialysis, anion exchange membranes segregate acid molecules into a purified zone. Typically, 80-95% of the acid is recovered with 70-95% of the metals removed.
The exit ports of the membrane stack are plumbed to a set of metering pumps. Except during the automatic refilling of the system, these metering pumps are the only moving components on the entire system. The metering pumps are used to control the solution flow rates. The exit ports of the recovered acid metering pump(s) are plumbed into the acid pickling tank, and in the case of the metal-rich, acid-depleted waste solution, the metering pump(s) are plumbed to final treatment.
The acid recycling system is a fully modularized unit. For installation, the pump on the acid recycling unit is plumbed to the working pickling tank(s) and a solenoid valve on the unit is plumbed to a pressurized water source. The system uses 115 VAC/ 20 AMP service and, upon delivery, can be plugged in and immediately utilized.
Implementing Acid Recycling
A stainless steel pipe manufacturer began to investigate Diffusion Dialysis for the recovery of their Nitric-Hydrofluoric acid pickling solution and for control of metal and contaminant buildup in their pickling bath. Immediate motivation was for the replacement of a resin-sorption recovery system that had been marginally successful at recovering their pickling acid and controlling the metals in their bath. Adverse maintenance and reliability issues associated with the resin-sorption recovery system were key motivators for investigating alternative technologies.
Resin Sorption technologies have occasionally been utilized for the recycling of stainless steel pickling baths. This technology relies on the sorption of acid molecules on an ion exchange resin bed. The process works by pumping contaminated acid into the bottom of the resin bed. Acid is sorbed by the resin particles and the partially de-acidified salt solution is collected from the top of the bed. Water is then pumped into the top of the bed, desorbing the acid from the resin and the recovered acid product is collected from the bottom of the bed. The above cycle is continuously repeated by alternately opening and closing a series of valves.
Acid recovery efficiency via resin sorption can vary between 40% to 90% per pass. Metal removal rates per pass can be as low as 25%. One reason for this low metal removal efficiency is due to the entrapment of process solution in the resin bed column. This entrapment hinders overall recycling efficiency because it requires multiple passes to achieve sufficient metal removal. With Diffusion Dialysis technology, significantly less waste by-product is producedtypically one-half to one-fifth as much as with resin sorption systems.
With the advent of significantly more durable ion exchange membranes in recent years, the life expectancy of the majority of the ion exchange membranes utilized in Diffusion Dialysis acid recovery can be up to 10-20 years, dependent upon the application. Typical ion exchange resin life in acid sorption systems varies between 2-5 years. Both technologies require very good prefiltration of the process solution prior to introduction into the recovery units.
To prove the effectiveness of the Diffusion Dialysis technology in removing metallic contaminants and in producing workable concentrations of recovered acid, a pilot study was performed at the customer’s facility, by in-plant personnel, on their working pickling solution. The pilot studies showed excellent results in removing metallic contaminants and generated a recovered acid permeate of high concentration for reuse. The acid depleted fraction following dialysis produced a solution which was rich in metal and weak in acid concentration.
The sizing of the Diffusion Dialysis system was based upon the volume of spent pickling solution previously produced, the rate of this production, and the efficiency of the Diffusion Dialysis process. A useful “rule of thumb” requires that, at a minimum, the volume of spent acid that was previously discarded be recycled once through the Diffusion Dialysis unit over the same period of time that it took to generate the spent acid.
A 600 GPD acid recycling system was installed directly on to the working pickling tank, as illustrated in Figure #1. Additions of virgin acid are made to replenish depleted volumes due to: consumption, drag-out, exhaust escape, and the minor amounts lost in the dialysis process.
Table #1 relates the Diffusion Dialysis performance results of this installation. Sixteen sample sets were taken over a three month period and averaged. Recycling efficiencies (in parentheses) were calculated by comparing the recycled acid and rejected metal concentrations to the initial pickling bath concentrations. The system is dialyzing about 500-600 gallons per day. A flow imbalance between the reclaim stream and reject stream can produce concentration increases, as seen in the increased nitric acid concentration in the reclaimed acid solution as compared to the initial nitric acid concentration.
Justifications and Benefits
Cost savings are a major justification for using Diffusion Dialysis for the recycling of pickling acids. Diffusion Dialysis acid recycling users obtain improved quality and reduced rework, often with reduced processing times. The following is a summary of benefits being derived from the implementation of acid recycling utilizing Diffusion Dialysis:
- Savings from reduced or eliminated disposal costs and reduced acid purchases
- Elimination of production down-time associated with the dumping and recharging of acid baths
- Minimization of direct operator contact with dangerous chemicals – reduced operator exposure
- Fully automatic operation, 24 hours per day, seven days per week, with very minimal operating costs
- Improved process control with consistent pickling rates, improved quality and minimized waste
- Improved environmental impact