Green Processing of Iron-Based Materials

Among the twelve principles of green science and technology is a desire that the production of materials be achieved from sustainable sources with minimal environmental impact, and preferably by recycling of waste materials. Currently, 10% of iron ore ends up as steel mill waste that amounts to 100 million tons annually. Furthermore, there exist numerous waste streams that are potential sources of iron including spent iron oxide catalysts (e.g. from styrene and ammonia synthesis), red mud from aluminum extraction, and coal mine sludge.

Iron is generally extracted from ores such as hematite by carbothermal reduction in a blast furnace at high temperatures. While the iron thus produced is a convenient starting material for production of iron-containing chemicals and ceramic, the synthesis of these materials by this route is unnecessarily energetically costly. For this reason, the Apblett research group has been developing alternative extraction processes of iron ores to give useful iron-containing chemicals and materials.

One approach that we have developed is the use of 2,4-pentandione to extract iron as a volatile complex from both conventional and unconventional ores and waste products.

In the case of conventional ores, a finely ground sample of hematite ore was received from Great Western Iron Ore Properties Inc. a company that has mineral claims on approximately 2,000 acres in the Tonto National Forest, in Gila County, about 8 miles north of Young, Arizona. This is arguably the best deposit of iron ore in the Southwest United States with an estimated to 284,200,000 tons of hematite as the primary iron mineral along with lesser amounts of a variety called specular hematite.

The ore sample was analyzed by X-ray fluorescence spectroscopy and found to be rich in iron oxide (88.1% Fe₂O₃) with small amounts of silica (9.8%), calcium (0.7% as CaO) and potassium (1.0 % as K₂O). X-ray powder diffraction identified the main minerals present to be hematite and quartz. Treatment of the ore with 2,4-pentanedione at 140°C removed the majority and yielded large red crystals of pure Fe(acac)₃. The extraction residue was pink in color and contained 1.5% iron that was tied up in an aluminosilicate mineral.

Hematite ore before (left) and after (right) extraction.

The extraction of iron from ores and other materials by reaction with acetylacetone can be very practical since the resulting iron(III) acetylacetonate, Fe(acac)₃, is extremely useful for production or iron containing materials. This includes chemical vapor deposition of iron oxide thin films and the sol-gel synthesis of iron-containing materials. Thermolysis of Fe(acac)₃ provides an facile route to nanoparticles of a large variety of iron-oxide and iron-containing materials. Iron acetylacetonate is also a highly efficient catalyst and reagent for a wide variety of organic transformations.

Excellent results were also realized utilizing Oklahoma red soil as an unconventional ore. Analysis by X-ray fluorescence showed that a sample of dark red soil collected in Payne County, OK contained 22.7% by weight of iron(III) oxide, making it a reasonable source for iron provided that the metal could be easily extracted. Therefore, the ore was treated with a 1:9 mixture of water:acetylacetone (V:V). After 48 hours at 140°C, the extractant had attained the dark red coloration of iron(III) acetylacetonate.

After filtration, it was discovered that the soil was now white but XRF spectroscopy showed that there was still a fair amount of iron oxide (6.9%) that remained unextracted. Nevertheless, a good yield of iron was obtained, indicating that iron-rich clay materials can be a useful source of metals. The white color of the extracted clay could make it suitable for use in manufacturing ceramics.

Treatment of the resulting Fe(acac)₃ with ammonium hydroxide produced Fe(OH)₃ that, upon heating, was converted to pure hematite. Alternatively, the Fe(acac)3 can be reduced to iron metal using a variety of reagents such as sodium borohydride, hydrogen, scrap aluminum, etc. For example, treating the dried hematite ore extract sodium borohydride produced a fine iron metal powder.

Red soil before (left) and after (center) extraction and the resulting Fe(acac)3 crystals.

The 2,4-pentanedione process has also been applied successfully to waste materials and was found especially useful for recovery of iron from coal mine sludge and from the red mud waste stream from the Bayer process for extraction of aluminum from bauxite.

Gluconic acid can be used as a greener replacement for 2,4-pentandione for the extraction of iron ore. It is derived from the oxidation of glucose and is completely non-toxic. Its complex with iron is extremely water-soluble and forms readily when a mixture of ore and aqueous gluconic acid are heated to reflux. The resulting iron gluconate can be converted to an iron and carbon mixture by heating in an inert atmosphere and could be further refined in a steel mill.