PHOSPHORIC ACID FROM ROCK PHOSPHATE AND SPENT HCL

Phosphoric acid (PA) is an important industrial chemical used as an intermediate in the fertilizer industry, for metal surface treatment in the metallurgical industry, and as an additive in the food industry. Wet-process PA (WPA) is produced by the attack of sulfuric acid (H2SO4) on phosphate rock (PR). Other wet processes use HCl and posterior solvent extraction technology. The corrosivity of phosphate ores, during the production of WPA, depends on two main factors: the chloride content and the interaction between HF formed in the WPA reaction slurry with SiO2, Al2O3, and MgO present in the ore. Many forms of corrosion, mainly localized, are encountered in the PA production plants and facilities such as erosion-corrosion (EC), selective corrosion, pitting, stress-corrosion cracking, intergranular corrosion, and corrosion at high temperature. Laboratory and plant corrosion tests were performed to recognize the corrosion types; EC measuring instruments were developed, built, and applied. The PA industry is spread out worldwide in Europe, Asia, Africa and America, including countries that operate PR mines and produces PA, phosphatic fertilizers, and phosphate-based products.

Phosphoric acid also known as other phosphoric acid (H3PO4) is an important acid, being used, vary widely in the ‘Fertilizer Industry (Phosphatic Fertilizers). Phosphatic fertilizers, certainly, affect the growth and contents of agricultural products. Thus, this product is of high level significance.

Phosphoric acid can be manufactured from phosphate rock because of following reasons:

a) Low manufacturing cost
b) Growth of synthetic detergent industry
c) Increased a high level demand in phosphatic fertilizers.

Commercial phosphate ores or “phosphate rocks have one property; its phosphatic content is a phosphate fluorine-calcium combination of apatitic structure. The phosphate rock producers express the content of their products as BPL grade, which is an abbreviation for ‘bone phosphate of lime’ for Ca3 (PO4)2. To convert BPL grades into P2O5 equivalent grade, divide by 2.183. In our country, this rock is found in Udaipur district in Rajasthan.

The manufacturing process of phosphoric acid, consists many processes. At the beneficiation stage, different techniques may be used to treat the same ore for removal of the gangue and associated impurities. This gives rise to further varieties in the finished ore concentrate product. It is clear, therefore, that phosphoric acid technology, having to rely on raw materials of great variety and of permanently fluctuating composition, has to readapt itself constantly.

Phosphoric acid can be produced from these raw materials via two major process routes: the wet process, using sulfuric acid attack and the electric furnace process, using electrical energy to product elemental phosphorus as a first stage. Because of current energy prices, the electric furnace process, although it can cope with lower grade phosphate rocks, has been largely abandoned except in cases where elemental phosphorus is needed. The wet process, which is the subject of this book, accounts for 90% of the current phosphoric acid production.

By its nature, wet process technology conserves most of the impurities found in the original phosphate ore, which are then included in the phosphoric acid produced. Consequently, the variety of the phosphage ores influences not only the process used but also the composition and characteristics of the phosphoric acid produced.

PROPERTIES

Phosphoric acid possesses following properties:-

1) Anhydrous phosphoric acid can be obtained in colourless crystals.

a) Melting Pt. = 42.35oC.

2) Commercial grades of phosphoric acid contain 50-90% of acid.

3) It is found in syrupy form when concentration is about 85%.

4) At 250oC it decomposes producing pyrophosphoric acid. On further heating metaphisphoric acid. or Glacial phosphoric acid.

5) It is a highly corrosive in nature.

2. a. It is clear colourless, odourless, sparkling liquid or transparent crystalline solid depending on concentration.

b. Sp.gr. 1.834 (18oC)