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Fertiliser Technology - Engineering

Q1:

In the manufacture of orthophosphoric acid by strong H2SO4 leaching wet process, keeping the reactor temperature above 100°C, results in the formation of undesriable

A CaSO4 .H2O and CaSO4 crystals

B pyrophosphoric acid

C metaphosphoric acid

D all (a), (b) and (c).

ANS:A - CaSO4 .H2O and CaSO4 crystals

In the manufacture of orthophosphoric acid by the strong H2SO4 leaching wet process, keeping the reactor temperature above 100°C can result in the formation of undesirable CaSO4·H2O and CaSO4 crystals. Explanation:

  1. Strong H2SO4 Leaching Wet Process: In this process, phosphate rock is treated with concentrated sulfuric acid (H2SO4) to produce phosphoric acid (H3PO4). This reaction typically occurs at elevated temperatures to enhance the dissolution of phosphate minerals in the rock.
  2. Formation of Calcium Sulfate: Phosphate rock often contains calcium compounds, such as calcium carbonate (CaCO3) and calcium phosphate (Ca3(PO4)2). When treated with sulfuric acid, these calcium compounds can react to form calcium sulfate (CaSO4), along with phosphoric acid.
  3. Temperature Considerations: Keeping the reactor temperature above 100°C can accelerate the reaction between sulfuric acid and calcium compounds in the phosphate rock. However, at higher temperatures, there is a risk of forming undesirable calcium sulfate crystals (CaSO4·H2O) and anhydrous calcium sulfate (CaSO4).
  4. Undesirable By-Products: The formation of calcium sulfate crystals and precipitates can lead to operational challenges and reduce the efficiency of the phosphoric acid production process. These by-products can interfere with equipment operation, reduce product purity, and increase production costs.
  5. Other Potential Products: In addition to calcium sulfate, other phosphorus-containing compounds, such as pyrophosphoric acid and metaphosphoric acid, may also form under certain conditions, further complicating the process and affecting product quality.
Therefore, maintaining the reactor temperature above 100°C in the strong H2SO4 leaching wet process can result in the formation of undesirable calcium sulfate and other by-products, impacting the efficiency and quality of orthophosphoric acid production.