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Historical Articles

April, 1953 issue of Plating


Chromic Acid Manufacture


Eugene L. Combs, Technical Service Division, Diamond Alkali Company

INTRODUCTION
All chromic acid produced in this country is made by reacting bichromate of soda with sulfuric acid. Bichromate of soda is the end-product of a process involving chromite ore, soda ash, lime, water and sulfuric acid. The process with improvements in equipment, controls and techniques has been used for over 25 years. The pictures show a modern adaptation of this process.

CHROMITE ORE
Chromite ore is available in many locations throughout the world, but there is a wide variation in grade. That which is most suitable for the production of chromium chemicals is referred to as chemical grade ore, and comes chiefly from the Transvaal area of the Union of South Africa. An average analysis of chemical grade chromite is as follows:

Chromic Oxide, Cr2Os3, 44.18%
Ferrous Oxide, FeO, 19.01%
Aluminum Oxide, Al2O3, 14.74%
Magnesium Oxide, MgO, 11.48%
Silica, SiO2, 3.53%

Fig. 1. One of several chromite ore stock piles required to assure continuous operation of one chromium chemicals plant. Chromite ore resembles crushed coal in its physical characteristics

The chromite ore is purchased on an assay basis through importers who have several sources of supply. The chromium chemicals producing companies maintain large inventories of the ore to offset the transportation hazards from the mines. Shipment from the mines to the port is by a single track railroad because of the terrain. When the demand for chromium chemicals is greatest, other materials are also in emergency supply and there is considerable competition for ore boats.

Beginning in 1946, there has been a steady gain in the world production of chromite ore. The Union of South Africa has more than doubled production. The Philippines have better than quadrupled their production while Cuba has experienced a slight decline. The production position of Southern Ethodesia, Turkey and New Caledonia has strengthened. Yugoslavia has shown a substantial gain in production while Russia has produced a large but fluctuating output. Other countries such as India, Sierra Leone, Greece, Cyprus, Bulgaria, Canada, Japan, Albania and Brazil have turned out lesser quantities of chromite ore.

The total world production of chromite ore is approximately 2 1/2 million short tons. The United States imports from 22 to 50 per cent of this amount with chromium chemicals production requiring from 7 to 18 per cent of the imported ore.

THE MANUFACTURING PROCESS
Grinding and Roasting
The chromic acid manufacturing process is essentially the conversion of chromite ore to sodium chromate solution, to bichromate of soda crystals, to chromic acid. The chromite ore, containing 42-50 per cent Cr2O3, is first dried and ground to a maximum coarseness of 100 mesh. This preparation increases the efficiency of the succeeding reaction step of the process.
The dried and ground ore is mixed with soda ash, and lime or ground filter residue from a previous operation. The ingredients are thoroughly blended in rotary mixers.

The mix is roasted in rotary kilns for about four hours in an oxidizing atmosphere, at a closely controlled temperature of 1094-1149° C (2000-2100° F).

Sodium Chromate Step
After leaving the kiln the roast is cooled and then transferred to a leacher where water is added and a nearly saturated solution of sodium chromate and sodium aluminate is obtained. The residue from leaching contains the iron, magnesium and silicon impurities in the ore. Part of this residue, called mud, is dried and ground for use in future mixes and the remainder is discarded.

Fig. 2. The kiln room showing the firing end of the roasting kilns in which the first chemical reaction required in the production of chromic acid takes place. Note the well-lighted control panel and working area. Good ventilation and ample space to encourage neatness is provided. The rotary kilos compare in size and design to the “mud” dryer shown in another photo Fig. 3. The residue or “mud” from the leaching step in the process is dried in this rotary kiln and passed into a crusher where it is reduced to a size suitable for re-use in the process

The leached sodium chromate-sodium aluminate solution is passed through a polishing filter to remove traces of suspended matter and then pumped into batch hydrolyzing tanks. Here sodium chromate solution is slowly added to the sodium chromate-sodium aluminate solution to crystallize hydrated alumina. The alumina is separated and washed on filters and either dried and marketed as such, or the wet filter cake is dissolved in sulfuric acid and converted to aluminum sulfate.

The purified sodium chromate solution is either sold as a 40° Baume solution, evaporated to produce anhydrous or hydrated sodium chromate crystals, or converted to bichromate of soda.

Sodium Bichromate Step
To produce sodium bichromate, the 40° Baume sodium chromate solution is treated with 66° Baume sulfuric acid. Approximately half of the sodium sulfate formed in this reaction is precipitated during mixing and the remainder drops out in the evaporator to which the solution is pumped for concentrating. When a concentration of 13.5 to 14.0 lbs Na2Cr2O7· 2H2O per gallon is reached, the solution is pumped into a battery of water-cooled crystallizers. When the temperature of a crystallizer falls to 35° C (95° F), the batch is centrifuged and the mother liquor is either sent back to the evaporators, the hydrolyzer, or sold in tank cars as a saturated solution containing 70 per cent by weight, Na2Cr2O7· 2H2O. The crystals are dried in rotary dryers, care being taken not to drive out the water of crystallization.

Fig. 4. The precipitated sodium sulfate is removed in these horizontal rotary filters. The precipitate is washed, dried and marketed Fig. 5. The crystal department where bichromate of soda crystals are produced. The crystals are removed from the mother liquor in large centrifugal filters, one of which can be seen to the left of Tank No. 2. The crystals are washed in the centrifuge, removed and transported to the crystal dryer and cooler by belt conveyors

Chromic Acid Step
In the chromic acid process, bichromate of soda crystals are mixed with either 66° Baume or fuming sulfuric acid in a kettle equipped with an agitator. Fuming sulfuric acid absorbs the water of crystallization while this water has to be evaporated if 66° Baume acid is used. The temperature of the mix is raised to 197° C (387° F) at which point the entire mass containing chromic acid and sodium bisulfate is in the molten state. Chromic acid, being heavier than sodium bisulfate, collects at the bottom and is drawn on and passed through water-cooled flaking rolls and packed in steel drums. The sodium bisulfate contaminated with chromic acid is run out into pans to solidify before being discarded.

The care exercised in the separation of the chromic acid from the sodium bisulfate determines the sulfate content of the chromic acid. The quality control laboratory analyzes the ingredients for each batch and the reaction time is adjusted, if necessary. The chromic acid is analyzed for CrO3 content and impurities. The individual drums of chromic acid are rapidly segregated by a sulfate analysis. All material showing sulfate higher than standard is rejected and reprocessed.

Fig. 6. Close-up of the crystal dryer and cooler. Careful control is exercised to maintain a product containing the correct amount of water of crystallization
Fig. 7. Fusion kettles used in producing chromic acid. The weights of bichromate of soda and sulfuric acid used in each batch are controlled by means of the scales shown in the background. The bichromate of soda crystals are added to the kettles by means of the pipe chute shown in the center of the picture. This chute revolves so as to service each kettle

One hundred pounds of bichromate of soda yields 62.4 lb of salable chromic acid, which is 93 per cent of the theoretically available amount. Loss in the bisulfate is about 5.5 per cent and up to 1.5 per cent is lost in dust and handling. The weight of the contaminated bisulfate produced and discarded is essentially equal to the weight of the bichromate of soda used.

It has been shown that the chromite ore contains iron, aluminum, magnesium and silicon in addition to chromium oxide. Of these impurities only aluminum is obtained in a salable form. The reason for using chemical grade ore is quite evident, since ores high in silica, iron, and magnesium mean the payment of freight on materials destined to be discarded as waste products.

The impurities in the chromite ore are removed in the manufacture of bichromate of soda. This material has been produced for sale in a highly competitive market for many years. Hundreds of man hours have been expended over these years to produce as pure a technical grade of bichromate of soda as it is chemically possible to achieve. This same bichromate of soda is used in the chromic acid process. The principal problems concerning the purity of chromic acid are in heating the kettle and in the separation of the sodium bisulfate and chromic acid layers. The temperature and time are controlled by instruments. A mistake at this point may cause the formation of undesirable complex chromium compounds. The correct separation of the layers controls the sulfate content. This separation is performed manually by a trained operator. The operator is double-checked by an analytical chemist and any chromic acid containing high sulfate is reprocessed.

Fig. 8. Chromic acid flakes pass directly from the flaker into the drums. An exhaust duct located behind the drum carries away the dust. The operators wear face shields, and respirators and keep their bodies well covered. Note how they fasten the bottoms of their trousers to protect against dust or splashing. A roller conveyor carries the filled drum to the weighing, sealing, and labelling department Fig. 9. A section of the air-conditioned quality control laboratory. A careful check of each step in the production of both chromium chemicals and the by-products is made by the laboratory staff

Chromic acid is sold in flake form because it is the most economical anhydrous form to produce. The flake is friable and, in shipment, some of it breaks up into a powder so that the farther the material is shipped, the more powder may be found in a drum.
Two container sizes are packed. One hundred pound drums are more convenient for the electroplating industry, while the 400 pound drum has proven more satisfactory for those industries using chromic acid to manufacture other chromium compounds.

USES FOR CHROMIC ACID
Metal Finishing Industry
Of the twenty million or more pounds of chromic acid produced annually, approximately 70 per cent is consumed by the metal finishing industry. This amount includes, in addition to chromium plating and anodizing, material used in the compounding of conversion coatings for zinc and cadmium surfaces, surface treatment products for aluminum prior to painting, a sealant for phosphatizing processes, and oxide control treatments for aluminum prior to spot welding. Other metal finishing uses which consume considerable chromic acid are chromidizing, chemical polishing baths, bright dips for copper and brass, and the stripping of the copper stop-off after selective carburizing. A unique application is the preparation of an aluminum surface to promote the adherence of a special porcelain enamel.

Pigment Industry
Approximately 20 per cent of the chromic acid is used by the pigment industry. Shades of yellows and greens not possible with bichromate of soda are produced by using chromic acid. Most important today is the production of a salt-free zinc chromate made possible through the use of chromic acid. This pigment is much in demand by the Navy as a corrosion inhibitor.

Fig. 10. The fast loading of a truck speeds the chromic acid to the consumer

The remaining 10 per cent is consumed in various ways. One use is in the manufacture of catalysts for oil refining, and for the vegetable shortening industry. Another use is in the manufacture of medicines such as Cortisone. It is claimed that chromic acid makes an excellent binder for refractory brick, especially well suited for open hearth bottoms. Chromic acid will harden and waterproof hydraulic cement. The oxidizing properties of chromic acid make this material most popular in the chemical industry. Rubber manufacturers use strong solutions of chromic acid for the fast cleaning of molds.

New Uses
A steady flow of patent grants covering new uses for chromic acid is cited. One of the most recent is the mixing of chromic acid with an unsubstituted ethylene polymer to produce a prime coat for bonding ethylene polymer coatings to steel. Without chromic acid in the composition, best adherence could not be obtained; with it, a new line of high quality coatings is now possible.

The unusual behavior of strontium with or in the presence of chromic acid solutions has been used in Europe to make a corrosion inhibitor for absorption refrigeration systems. Strontium carbonate is reacted with chromic acid to form strontium chromate. The strontium chromate is added to the system in excess of the desired amount and maintains a maximum concentration of 0.2 per cent chromic acid, the inhibitor concentration being measured as chromic acid.

The wide range of applications for chromic acid is not only a function of the chemical properties of the material, but is also due to its purity and its physical properties.

In conjunction with developments of new techniques in producing chromic acid, and in quality control methods, the manufacturer has kept abreast of developments in material handling methods. Modern methods of handling material mean better service to industry.

Chromic acid will continue to serve the needs of the metal finishing industry regardless of the development of new uses. Recent plant improvements and expansions are aimed at anticipated future requirements of industry. Production is geared to research and will grow with developments.

ACKNOWLEDGMENT
The author wishes to express his appreciation to Mr. A. M. Waller, Manager of the Kearny Plant, for procuring the photographs used with this article.



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