chemical recovery systems for aluminium finishers - Eco-Tec

TECHNICAL PAPER 129 

CHEMICAL RECOVERY SYSTEMS FOR 

ALUMINIUM FINISHERS 

Paul Pajunen, P. Eng., Group Leader Metal Finishing, Eco-Tec Inc., Pickering, Ontario 

Presented at Workshop on Cleaner Production in the Metal Finishing Industry, 

Taipei, Taiwan, April, 1999 

Introduction 

Anodising processes use water-based chemicals that can be treated in a fairly straightforward 

manner. While most municipalities require that the water leaving the plant be adjusted to the 

correct pH (acidity/alkalinity) range, many now also specify removal of suspended solids. After 

pH adjustment, solids can be filtered out of the water and disposed of in a landfill as a nonhazardous 

waste. Some anodisers have been able to send this solid waste to companies that 

convert it into aluminium sulphate liquid (alum). 

Although anodising wastes are relatively easy to treat, many plants use recovery equipment to 

reduce costs. This is due, in part, to the large amount of solid waste (aluminium hydroxide 

sludge) that etching and anodising generates (Table 1). Recycling reduces this waste, lowers 

chemical costs, and, frequently, improves product quality. 

Table 1 - Solid waste from an Etch/Anodise operation 

Aluminium dissolved in etching 

(1.5 mil etch) 

Aluminium dissolved in anodising 

(0.7 mil anodic film) 

Waste sludge produced 

(15% w/w solids) 

Waste sludge produced when 

etch regeneration is used 

Waste reduction with 

etch regeneration 

9.2 kg 

0.93 kg 

196 kg 

29 kg 

85% 

Note: Based on 93 m 2 of anodised surface area weighing about 182 kg. 

This paper will provide a general review of a number of recovery techniques. Detailed 

discussion will review three recovery techniques that are commonly used to regenerate the etch, 

purify the anodising acid, and recover bright dip solution from rinsewater. 

I. Recovery systems - general discussion 

Most recovery technologies can be grouped as being: 

SM/Technical Paper 129 03/99


- concentration based, or, 

- separation based 

For example, colouring bath chemicals are rinsed from anodised parts with water. The dilute rinse 

water that is formed contains metal salts from the colouring solution. A recovery system here 

might work by concentrating the rinse water until the volume was small enough to allow it to be 

added back to the colouring tank. This would be a concentration based process. In contrast, 

dissolved aluminium gradually contaminates anodising solution. A system that removes 

dissolved aluminium would be an example of a separation based process. 

Concentration based processes 

The simplest recovery systems are often concentration based. A common pitfall with these 

systems is the failure to consider contaminant buildup. Better designs include a purification unit 

with the concentrating equipment. This ensures that recycled chemicals will not degrade the 

process solution. Concentration based processes include: 

Evaporation 

The popular atmospheric evaporator employs the humidification principle. Liquid is pumped 

through a heat exchanger and sprayed down over a packing (plastic rings provide a high surface 

area) material. Air is drawn up through the packing evaporating the water. The design is simple, 

inexpensive, and well suited to smaller applications where the final concentration of the liquid is 

not too high. The only disadvantage of this design is that energy requirements are somewhat 

high, and there is an air discharge to consider. 

Boiling evaporator designs are also very effective, particularly for higher solution concentrations 

and capacities. They are somewhat more complex and considerably more expensive than the 

atmospheric evaporator described above. These units are often operated under vacuum to 

reduce the boiling point of the solution and improve heat transfer. 

Reverse Osmosis (RO) 

Reverse osmosis uses high pressure to force water through a semi-permeable plastic film or 

membrane. RO produces a concentrated metal salt stream by rejecting the dissolved salts. The 

membranes are susceptible to fouling so pretreatment of solutions is critical to success. The major 

operating costs for RO are membrane replacement and energy. Although RO systems have 

been used by the plating industry to some degree, they are not widely used by anodisers. 

Separation based processes 

Separation technology can be used in conjunction with a concentrating system (to recover pure 

bath chemicals from a rinse, for example) or on its own (to purify a process bath). 

Acid Sorption 

Certain ion exchange resins have the ability to sorb strong acids (sulphuric, hydrochloric and nitric 

acid) while rejecting metal salts of these acids. Water is used to recover the acid from the resin. 

Recovered solutions contain most of the unused, or free acid, with up to 90% of the metals 

removed. The most unique feature of the sorption process is that no chemicals or significant 

amount of energy are required. 

The acid sorption process has been packaged into a standard, skid-mounted piece of equipment 

called an acid purification unit (APU ® ). The APU has been used to purify acid-based process 

baths and is used in conjunction with other purification and concentrating equipment as part of 

more complex systems. 



Acid sorption technology employs 

specially treated ion exchange 

resins that have the ability to sorb 

free (unused) mineral acids while 

rejecting the salts (e.g., sodium 

sulphate) of these acids. The most 

unique feature of these resins is their 

ability to release the acid with a 

simple water wash. 

Sorption 

resin 

particle 

Water 

Sulfuric 

acid 

Sodium 

sulfate 

Diffusion Dialysis (DD) 

Diffusion dialysis is similar to acid sorption except that an anion exchange membrane is used 

instead of sorption resin. The acid passes through the membrane into the water stream while the 

metal salt impurities stay behind. A multitude of these membranes are held together in a unit that 

resembles a filter press. As with RO, pretreatment is very important for this process as 

membranes account for a large portion of the system cost. 

Crystallisation 

Crystallisation depends on variations in the solubility of metal salts. Temperature and 

composition can often be adjusted to cause a metal salt to crystallise from a saturated solution. 

The salts are filtered, washed, and, frequently, sold. 

Other processes 

Some technologies span both the concentration and separations categories: 

Ion Exchange (IX) 

Ion exchange resins can replace cations with hydrogen (H + ) or anions with hydroxyl (OH - ). In a 

water deioniser, anion exchangers follow cation exchangers with the result being the generation of 

pure water. Cation exchangers are regenerated with sulphuric or hydrochloric acid while sodium 

hydroxide is used to regenerate an anion exchanger. While the most common use of ion 

exchange is water deionisation, a more attractive application in the metal finishing industry is the 

recovery or purification of finishing solutions. A special form of ion exchange called reciprocating 

flow ion exchange, or RECOFLO ® , is particularly well suited for chemical recovery applications. 

Electrodialysis (ED) 

In the ED process, electricity is used to drive ions across ion exchange membranes. A 

combination of cation and anion exchange membranes can be used to deionise a solution and 

recover a concentrate. Solutions containing low levels of metals (such as rinsewaters) can be 

concentrated to allow metal salt recycle. The process is better suited than RO to applications 

where a high degree of concentration is required. To date, the major industrial applications of 

electrodialysis have been in the field of water purification. 

II. Recovering systems - specific examples 

Aluminium anodising is normally done by immersing parts in sulphuric acid and applying current. 

However, there are a number of other important steps involved. Many of these processes 

present recovery opportunities. Three recovery techniques described below are commonly 

used. 



Caustic etch recovery 

Extruded parts show die lines and minor surface imperfections which must be removed in the etch 

tank. A hot solution of sodium hydroxide removes a thin layer of aluminium and creates an 

appealing matte surface finish. 

Etching is caused by a reaction between the aluminium and caustic soda that produces sodium 

aluminate and hydrogen gas as follows: 

etching reaction 

2Al + 2NaOH ----> 2NaAlO 2 + 3H 2 (gas) 

The etching process is typically responsible for 80-90% of the aluminium in the waste treatment 

system. Chemical stabilisers (complexing agents) are added to prevent the aluminium from 

precipitating out in the etch tank. The additives thicken up the solution to the point where enough 

liquid is carried out on the parts to keep the aluminium level from building up in this "never dump" 

etch. Water is used to rinse the etching solution off the parts. The rinsewater carries dissolved 

aluminium and caustic to the plant waste treatment system. 

If stabilisers are not used and the sodium aluminate concentration is allowed to rise too high, it will 

hydrolyse to produce alumina tri-hydrate (Al 2 O 3 

. 3H2 O), thereby liberating free caustic soda. 

hydrolysis reaction 

2 NaAlO 2 + 4H 2 O ---> 2 NaOH + Al 2 O 3 

. 3H2 O 

This reaction, known as the Bayer process, is used in the primary aluminium industry to make 

alumina. If not properly controlled, it leads to an accumulation of a rock-hard aluminium hydroxide 

scale in the etch tank. By seeding the etch solution with alumina crystals in a separate 

crystalliser tank, it is possible to regenerate the etch solution without having scale buildup. 

The basic operation of a regeneration system is such that etch solution is recirculated 

continuously between the etch tank and the crystalliser tank. Hydrated alumina crystals form in a 

slurry section of the crystalliser and settle out in the clarification section. Regenerated etch 

solution, with reduced aluminium and increased free caustic levels, flows back to the etch bath 

directly from the top of the crystalliser. Alumina crystals are withdrawn periodically from the 

bottom of the crystalliser and dewatered in a filter press. 

Over the past ten years many large architectural anodisers have installed regeneration systems 

based on this process. Regeneration can reduce a plant's solid waste by over 80% while 

lowering caustic chemical (and neutralization) costs by over 70%. The alumina crystals which are 

removed can be put to a variety of uses as an alumina substitute. Although these systems are 

relatively expensive to install, larger plants can recover their costs within two or three years. 

Crystallisation systems such 

as the one shown here are 

used to continuously remove 

aluminium from caustic etching 

solutions. The aluminium is 

converted to a form that can be 

readily sold and the caustic is 

returned to the etch. The Alrec 

process can be used where a 

heavy matte surface finish 

is desirable. 



One drawback to a regenerated etch relates to its lower aluminium levels. As high aluminium 

levels promote a more matte finish, there were initially some concerns that a regenerated etch 

would not yield a suitable finish. With several systems now in operation in North America, a 

regenerated etch is widely regarded to produce a finish that is slightly less matte than a "neverdump" 

finish but satisfactory for most applications. A new high performance etching process has 

been introduced that is compatible with regeneration systems. This process, known as Alrec, 

allows an anodiser to achieve a premium heavy matte finish while reducing etch time, chemical 

costs, and waste generation. 

Sulphuric acid anodising purification 

The anodising operation itself represents an excellent opportunity for purification as the dissolved 

aluminium levels must be kept quite low. By the time the aluminium level in the acid reaches 15- 

20 g/L, the solution is decanted or dumped. In addition to eliminating a waste problem, continuous 

purification can enhance the uniformity of the anodised film. 

Operating an anodising bath in a dump/decant manner presents a number of potential problems. 

These become apparent due to the fact that there exists a delicate balance within an anodising 

bath - namely, the relationships between the electrical resistance (caused by the formed oxide 

coating and the anodising solution conductivity), the voltage being applied, and the desired 

constant current condition. The electrical resistance increases relative to the thickness of the oxide 

coating and to the increasing aluminium concentration in the anodising solution. To compensate for 

this increased resistance, the rectifier voltage must be increased in order for the current to remain 

constant. Adding in other variables such as bath temperature, degree of solution agitation, and 

sulphuric acid concentration can result in upsets and potentially lead to a decline in product 

quality. 

Maintaining a consistent, low aluminium concentration removes or minimises a variable that can 

affect the balance between resistance, voltage, and current. 

Controlling the aluminium concentration and recovery of sulphuric acid for continued use in the 

aluminium finishing industry has been practiced for a number of years. The end result is ensuring 

a consistent, predictable bath operation leading to cost savings and improved product quality. 

A popular method of acid recovery employs a process called acid sorption (APU). It is estimated 

that close to 400 systems using this technology are currently in operation worldwide. 

The APU is a pre-assembled, 

skid mounted device that is fully 

tested, with resin, prior to 

shipment. The resin column is 

mounted on an epoxy coated 

steel frame with process valves, 

internal piping and a control 

panel. The panel includes a PLCbased 

control system. 

Bright dip recovery 

Concentrated phosphoric acid solutions, usually with additions of nitric acid, diammonium 

phosphate and copper, are used to chemically brighten aluminium parts. After brightening, the 

adhering solution must be rinsed off immediately with water. Due to the high acid concentrations 



and viscosities of bright dip baths, carryout of bath solution on the parts is typically 3-4 times 

greater than from an anodising tank. While aluminium contamination of the bath is rarely a 

problem, there is a substantial loss of bath chemicals. 

Most plants collect the rinsewater as a 35% solution for resale as fertiliser. The seasonal and 

regional variations in demand for the rinsewater reduce the value to between 10-20% of the 

original chemical cost. 

The rinse can be reconcentrated to bath strength with a vacuum evaporator; however, a 

purification step must be employed to prevent aluminium buildup. A cation exchanger (called a 

DCU) removes about 90% of the aluminium before the solution flows to the evaporator. The 

DCU is regenerated with sulphuric acid and the waste, containing sulphuric acid and aluminium, is 

processed with an APU. Purified sulphuric acid is returned to the DCU for use in the next 

regeneration cycle. 

This phosphoric acid 

recovery system was 

designed to recover over 

1800 tonnes of 80% acid per 

year. The vacuum 

evaporator is located on the 

right side. To the left are the 

DCU and APU. The cooling 

tower was roof mounted. The 

total area required for this 

system was 185 square 

metres. 

The aluminium sulphate waste, or byproduct, produced by the system can be neutralised in the 

plant waste treatment system or concentrated by evaporation and sold as liquid alum. Typical 

results are given in Table 2. 

Table 2 - Phosphoric Acid Bright Dip Recovery - typical results 

Stream 

Phosphoric acid 

(g/L) 

Aluminium 

(g/L) 

Bright dip bath 1340 20-40 

First (feed) rinse 150-200 3-6 

Third (final) rinse


Conclusion 

With the increased focus on environmental concerns in all facets of business, anodisers can look 

at a number of opportunities in their plants that will reduce process chemical costs, reduce waste 

treatment chemical and labour costs, and, in many cases, enhance product quality. Recycling is 

an environmentally pro-active step that demonstrates a responsible corporate image to your 

customers, employees, and to the local authorities. 

References 

1. Pernick, J. “Problems in Hardcoat Anodising”, Plating and Surface Finishing Magazine, p. 32, 

(June, 1988). 

2. Dejak, M., “Acid Purification and Recovery using Resin Sorption Technology - An Update”, 

presented at the AESF/EPA Pollution Prevention Conference, Orlando, Florida (1994) 

3. Brace, A.W., “Anodic Coating Defects - Their Causes and Cure”, Technicopy Ltd., 

Gloucestershire, England (1992) 

4. C. Brown, C. Fletcher, "The Recoflo® Ion Exchange Process", Ion Exchange for Industry, M. 

Streat, Ed., Chichester, UK: Ellis Horwood Limited (1988) 

5. C. Fletcher, V. Pace, "New Performance Standards for Demineralization set by Recoflo® 

Technology", presented at the American Power Conference, Chicago, Illinois (1995) 

About the Author 

Paul Pajunen has, for the last 12 years, been with Eco-Tec of Pickering, Ontario, Canada, in 

various roles as process engineer, customer service engineer, and sales/marketing 

representative. He currently leads Eco-Tec's marketing efforts as it pertains to the electroplating 

and aluminium metal finishing sectors. Mr. Pajunen holds a degree in chemical engineering from the 

University of Waterloo, is a registered professional engineer in the Province of Ontario, is a 

member of the American Electroplaters and Surface Finishers Society, Inc. (AESF), is a member of 

the Aluminium Anodisers Council (AAC), and is involved with the Metal Finishers Suppliers 

Association (MFSA), Canadian division.

chemical recovery systems for aluminium finishers - Eco-Tec

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