Physical and Chemical Treatment of Boron Ores

Physical and Chemical Treatment of Boron Ores

A background on the history of boron is useful for those not familiar with the industry. More than 4,000 years ago, goldsmiths may have found the first commercial use for borax as a flux in making jewelry. Boron is found commercially in Argentina, Bolivia, Chile, China, India, Iran, Kazakhstan, Peru, Russia, Turkey, and the United States. The United States and Turkey are major supplies in the world since large scale mining began in the 1800’s and have large high grade deposits of tincal and colemanite.

Boron does not occur in nature as an element, but is found combined in compounds with oxygen called borates or boron oxide. There are only about ten borates that have been used commercially and these are found in sedimentary deposits. Probably the most well known is borax, which is a white crystalline substance chemically known as sodium tetraborate decahydrate (Na2B4O7·10H2O). This paper will discuss the surface treatment and the application of chemical or physical enhancements to tincal, kernite, ulexite, datolite, brines, and colemanite.

Tincal

In the United States, the first borax was discovered in California in 1856 embedded in mud according to the Mineral Resources (1919). A large deposit of crude borax named tincal was found in 1925 and production began in 1927. The underground production was transferred to a surface mine and renamed the Boron mine in 1957 and is now part of United States Borax, Inc. The tincal is easily soluble and processing consists of dissolving in hot water and separating from the clay material. When sodium hydroxide is added to the hot borax solution, triclinic crystals of sodium metaborate tetrahydrate form when the solution is heated below 53.6°C. Sodium pentahydrate rhombohedral crystals are produced when the solution is cooled to 24° C in the presence of sodium bicarbonate.

Na2B4O7·10H2O + H2SO4 ⇒ 4H3BO3 + Na2SO4 + 5H2O……………………………………….(1)

The sodium borates react with aqueous ammonium chloride to precipitate ammonium pentaborate as follows:

5Na2B4O7 + 10NH4Cl ⇒ 10NaCl + 4NH4B5O8 + 6NH3 + 3H2O………………………………………(2)

Kernite

Kernite is found in large amounts associated with tincal at the Boron mine. The ore can be mined and crushed with water. After a period of time the majority of the kernite hydrates to crude borax. The borax produced can be processed to produce a higher purity borax or boric acid by equation 1. The following equation shows an example of kernite being processed into boric acid:

Na2B4O7H2O + H2SO4 + H2O ⇒ 4H3BO3 + Na2SO4………………………………………………..(3)

Natural Brines

Dissolved sodium borates occur naturally in brines at Searles Lake, CA, where a plant has been in operation since 1917 and is now owned by North American Chemical Corp. Brine is pumped into carbonation towers where carbon dioxide obtained from calcining limestone is pumped into the towers to precipitate sodium carbonate. The borax-rich brine is blended with lake brine to raise the pH and cool the brine to 38°C. Borax seed crystals are added to precipitate crude sodium tetraborate pentahydrate.

Another process used at Searles Lake requires evaporation. Tetraborate reacts with alkaline carbonate to form sodium metaborate as follows:

Na2B4O7 + Na2CO3 ⇒ 2Na2B2O4 + CO2…………………………………………………………………….(4)

The metaborate is very soluble and will not crystallize under the same conditions as tetraborate. A sodium bicarbonate-borax mixture causes the following reaction:

2NaHCO3 + 2NaB2O3 + O2 ⇒ Na2B4O7 + 2Na2CO3 + H2O…………………………………………..(5)

Datolite

In Dalnegorsky, a remote township in Russia, datolite (CaO·B203·aSiO2·bH2O) ore containing between 6% and 12% boron oxide is mined, Primorsky (1992). The ore is crushed and blended to a concentration of 8.6% boron oxide. The datolite is not soluble and the ore is washed to remove gangue after crushing.

Decahydrate borax is also produced by the reaction of boric acid with sodium carbonate (soda ash) as follows:

4H3BO3 + Na2CO3 + 4H2O ⇒ Na2B4O7·10H2O + CO2………………………………………………(6)

Elemental boron is manufactured by reacting boric anhydride with an alkali metal such as magnesium as follows:

B2O3 + 3Mg ⇒ 2B + 3MgO………………………………………………………………………………………(7)

The purity of the elemental boron can be improved by using a hydrochloric acid leach.

Ulexite

Some 35 playas are known to contain borates in Argentina, Peru, Bolivia, and Chile at altitudes of 760 to 4,600 meters above sea level. In Chile at the Salina of Surire, Quimica E Industrial Del Borax Ltda., or Quiborax, mines ulexite (NaCaB5O9·8H2O) (C.A.S. number 1319-33-1) 1,680 meters above sea level. The deposit is located southeast of Africa.

NaCaB5O9·8H2O + 2H2SO4 → 5H3BO3 + NaHSO4 + CaSO4 + 2H2O…………………………………..(8)

United States

Borate crystals were discovered in the 1800’s in walls of a canyon in Death Valley, CA, and were assayed and named colemanite. The material is insoluble in water and requires a different refining process from the cotton-ball ulexite found in the Valley floor. The colemanite ore was converted to a hot solution of sodium borate by treatment with soda ash. The calcium carbonate precipitated to the bottom and borax crystallized on the wall of the vat as the solution cooled. The sodium metaborate was treated with carbon dioxide to produce borax.

HCl + Ca2B6O11·5H2O + 2H2O → 6H3BO3 + 2CaCl2………………………………………………(9)

CaCl2 + H2SO4 → CaSO4 + 2HCl……………………………………………………………………………(10)

Turkey

There are three types of borates, colemanite (2CaO·3B2O3·5H2O), ulexite (CaO·Na2O·3B2O3·5H2O), and tincal (Na2O·2B2O3·10H2O), commercially exploited in Turkiye. These hydrated borates are found in the west part of the country and they are usually processed at the mine sites. All borate production is carried out by ETIBANK which is the largest state-owned mining and metallurgy company in Turkiye. ETIBANK mines and processes the borates in four main areas, Emet (colemanite), Bigadic (colemanite and ulexite), Kestelek (colemanite and ulexite) and Kirka (tincal). The company also refines tincal and colemanite ores and concentrates in Kirka and Bandirma plants. The latter which has port facilities is situated near the sea of Marmara.

Update of Research on Colemanite Beneficiation

OPTICAL SORTING. Sorting methods can be classified as two different methods; hand picking and optical sorting. Although hand picking methods have been used in some mineral processing plants in Turkiye for certain sized ore, such as for coarse colemanite concentrate (plus 25 mm), optical sorting methods could replace hand picking methods in the near future because of labor cost increases.

ATTRITION SCRUBBING. Attrition scrubbing methods are highly efficient for removing surface films, such as clay coatings, which are the major contaminant of borate ores and they can be used for the concentration of colemanite. For test purposes a specially designed scrubbing unit and for smaller amounts of sample, a Denver flotation cell, were employed in one study, Yilduran (1985). Two colemanite samples were studied, called the low and high grade ores contained 24.5% B2O3 and 26.8% B2O3 by weight, respectively. The concentration of the low-grade ore was completed in six successive stages. At the end of the third of stage scrubbing a concentrate with just under 40% B2O3 content was obtained but the recovery was very low at only 48%. Nevertheless, after the removal of this concentrate the process was continued with the middling fraction. The desired level of B2O3 was not obtained by further scrubbing of the middle size fraction although removal of the clay particles from the ore was virtually complete. The minus 53 mm fraction was removed from the slurry as tailings after completion of each stage. If the plus 53 mm fraction was taken as the concentrate, the recovery and grade would be 79.7% and 34.9% B2O3 respectively. This is below what would be commercially acceptable and it can be concluded that for the treatment of ore of this type attrition scrubbing as the only separation process is not technically viable.

DENSE MEDIUM CONCENTRATION. Dense medium separation, heavy medium separation, or sink-float separation are based on differences in mineral densities. The major difference is the medium of concentration. Rather than separation in water or air, in dense medium separation, the separation is in a medium of density higher than that of water.

FLOTATION. Flotation is one of the well-established physio-chemical separation processes widely applied in mineral concentration and recovery operations. The froth flotation process primarily relies on the fact that hydrophilic particles are wetted by water, whereas hydrophobic particles are wetted by oils and air bubbles; therefore if the air bubbles are introduced into an aqueous slurry, the bubbles adhere to the hydrophobic solid particles. As a result, air-solid aggregates are carried to the surface, forming a forth layer; this explains the name, froth flotation.

If the colemanite ore includes calcium carbonate, differential flotation of colemanite from calcium carbonate may be difficult to achieve. A short series of size reduction tests would therefore be necessary to achieve liberation, and enable an initial assessment of the potential for froth flotation to be made. It might also be necessary to remove the clay particles from the media before flotation, as their occurrence in large amounts in flotation circuit increases the retention time as well as the amount of reagents used. In order to handle a reasonable feed for flotation, attrition scrubbing and desliming may be necessary after size reduction, Yarar (1971), (1973).

 

physical and chemical treatment of boron ores