Acid Purification

Build up of metals and organic materials during acid use can shorten the life of your acid and negatively impact your costs of disposal. If the salts and organics could be removed, the acid could be reused, extending the life of the acid and reducing disposal costs.

DOWEX™ ion exchange resins and DOWEX OPTIPORE™ polymeric adsorbents have proven useful to a number of customers for this very purpose. DOWEX ion exchange resins have high capacities for many metals and mineral salts that can accumulate during use. DOWEX OPTIPORE polymeric adsorbents also can be effective in removing undesirable color components and soluble organic compounds from a variety of mineral acids.

Composition
DOWEX ion exchange resins and DOWEX OPTIPORE polymeric adsorbents are made from highly cross linked polymers of styrene and divinyl benzene. They are insoluble in concentrated acids or bases or organic solvents.

Ion Selectivity
DOWEX ion exchange resins remove salts and metals based on the selectivity of the resin for the salts. Metals such as sodium, potassium, calcium, iron, nickel and zinc, to name only a few, can be effectively removed as cations. Other metals form anionic complexes (42KB PDF) in strong acid solutions and can be affectively removed using anion exchange resins such as DOWEX 21K XLT resin.

Dow has a number of ion exchange resins with different selectivity options for a variety of applications. Additional considerations might be the level of other contaminants present and the type of equipment to be used. For additional information, see Products by Application and Type via the Periodic Table of Elements.

Concentrated HCl and Iron Removal
In the presence of high concentrations of HCl or brine, ferric chloride takes-on additional chloride ions to become an anionic complex. This complex binds strongly with the strong base anion exchange resin, making it possible to achieve HCl product with very low iron content. The ferric chloride complex quickly decomposes in fresh water to uncharge iron chloride and is quickly eluted from the resin with just water.

Organics Removal
The removal of organic materials from acids can be performed with DOWEX OPTIPORE L493 polymeric adsorbent.

In one application, 33 to 35% hydrochloric acid, containing methylene chloride and chloroform has been purified to the customers' desired purity (1).

The adsorbent was regenerated with steam, restoring the adsorption capacity for continued use. A 32% waste hydrochloric acid stream containing benzene and chlorobenzene was also purified with polymeric adsorbents and regenerated with steam (1).

Hydrochloric acid has also been purified of silicon containing impurities with polymeric adsorbents (2). In this instance, the regeneration can be performed with either heptane, methanol, NaOH or NaOH in methanol.

The selection of the right polymeric adsorbent will depend on the organic materials present, the type of regenerant that is compatible with your process and the acid application. Contact us for help.

Acid Retardation
Acid retardation is a process which is used to separate acids from their corresponding salts. The feed solution is passed over a strong base anion exchange resin to separate the components. The resin has a greater affinity for the acid than for the salts, resulting in a chromatographic separation of salts from the acid whose movement is "retarded" by the greater affinity. The acid is then displaced from the resin by a fresh water pulse. In some cases the acid can be recovered in significantly higher concentration than the original feed solution.

The acid retardation process occurs due to the selective absorptivity of the hydronium ion on the anion exchange resin. The concentration of the acid inside the resin is much greater than in the external solution. Because the salt components of the influent stream do not diffuse into the resin phase, the mineral acid can thereby be separated from its salt. Theoretically, this phenomena is called a Donnan membrane effect.

DOWEX 1x4 50-100 mesh or DOWEX 21K XLT are recommended for this application or the DOWEX RETARDION 11A8 resin.

Acid retardation is generally carried out in metal handling industries, where heavy metal ions are present in the process acids. In the metal plating industry, resins can be used to recover and recycle the acid used in pickling sheet metal, which would normally serve as a pollution problem and an economic expense.

Typical Separations Using Acid Retardation:
HNO₃ and Fe(NO₃)₃: Nitric acid is commonly used to pickle sheet steel. Acid retardation allows the remaining acid solution to be separated from the iron cations for reuse. Flow rates of 1 - 2 m/hr (0.4 - 0.8 gpm/ft²) are typically used for the separation and almost 90% of the acid can be reclaimed at approximately half of the original concentration.

H₂SO₄ and FeSO₄: Sulfuric acid is a common pickling agent for the steel industry. In the waste pickling liquors a mixture of sulfuric acid and ferrous sulfate is present, which can be reasonably separated using acid retardation. There is a great deal of flexibility in the type of separation that can be obtained by changing one or more operating parameters e.g. flow rates of 1.5 - 5 m/hr (0.6 - 2 gpm/ft²) and temperatures of 25 - 65°C (77 - 150°F) can be used.

HNO₃ and NH₄NO₃: Nitric acid and ammonium nitrate systems occur in many areas of chemical processing. Although both are relatively inexpensive products, the nitrate ion is becoming an increasing pollution concern, so by recycling the nitric acid, the amount of nitrate ions which are emitted into a plant waste stream are greatly reduced.

HCl and NaCl: The separation of hydrochloric acid and sodium chloride can be achieved using acid retardation techniques. By reducing the amount of hydrochloric acid to be discharged, the amount of neutralization and sodium chloride which would be necessarily discharged into surface waters is reduced.

Dilute HCl and FeCl₂
In some cases, hydrochloric acid is used to pickle sheet steel and acid retardation can be used to reclaim a portion of the acid for recycle. The acid concentration available for recycle may be up to 80% of the original concentration.

DOWEX Resins

Product	Description	Type	Matrix	Total Exchange Capacity* eq/l (kgr/ft3)	Available Forms
DOWEX OPTIPORE L493	A general purpose polymeric adsorbent for the concentration of a wide variety of chlorinated and organic solvents from water.	Adsorbent	Styrene-DVB, Macroporous
DOWEX 1X4	Fine mesh resin for fine chemical and pharmaceutical column separations.	Fine Mesh	Styrene-DVB, Gel	1.0 meq/ml	Cl-
DOWEX 21K XLT	Uniform particle size, high capacity, strong base anion exchange resin for mineral processing applications.	Type I Strong Base Anion	Styrene-DVB, Gel	1.4	Cl-
DOWEX Retardion 11A8 50 - 100 mesh	Amphoteric resin containing both weak acid cation and strong base anion functionality within the same resin.	Amphoteric resin	Styrene-DVB, Gel	1.1 meq/ml (24.1 kgr/ft3) 1.2 meq/ml (26.3 (kgr/ft3)	-COOH and -N(CH3)3

Additional downloadable information is available on DOWEX Ion Exchange Resins – Powerful Chemical Processing Tools. (164KB PDF) References 1. R. Hellmig, GDR Patent DD249 190 A1, September 1987. 2. H. Cronin, W. Evanko and A. Malekadeli, US Patent 5,330,735, July 1994.