Mineral Acids Manufacture: HCl, HF & H2SO4. Chapter- 3

Chapter- 3 Mineral acids manufacture: HCl, HF & H2SO4 Hydrochloric acid (HCl) • Is the 26th rank highest volume chemically produced acid. • Qualities up to 28%, 31%, 35% and 37% manufactured as an acid solution of HCl gas in water. • Manufacture of HCl: involves two steps a) Manufacture of HCl gas b) Absorption of HCl gas in water Methods of manufacture of HCl gas: i. From NaCl – is called salt cake method The process is carried out in cylindrical shaped muffle furnace at elevated T 0 (150 0C) with H2SO4. NaCl + H2SO4 → NaHSO4 + HCl↑ The temp. of the muffle furnace is further increased to 550 0C and the NaHSO4 formed is allowed to react with the remained NaCl; NaHSO4 + NaCl → Na2SO4 + HCl↑ 1 The Salt-Sulphuric acid process The reaction between the NaCl and H2SO4 is occurred in two endothermic stages 2 Cont’d… ii) Combustion of H2 with Cl2: carried out in pipe made of silica by filling with H2 and then Cl2 is introduced through the pipe by a small nozzle . H2 + Cl2 → 2HCl 3 Cont’d… The initiation, propagation and termination chai reaction are as follows, Initiation: Cl2 + heat → 2Cl• Propagation: Cl• + H2 → HCl + H• H• + Cl2 → HCl + Cl• Termination: Cl• + Cl• → Cl2 + Heat H• + H• → H2 + Heat H• + Cl• → HCl +Heat • A large amount of heat is developed both from chain propagation reactions and from chain termination processes. The continued renewal of the chain propagators by thermal route is ensured over the long term. In brief reaction between H 2 and Cl2 to produce HCl is a chain reaction with a high quantum yield. 4 b) Absorption of HCl gas in water ― Its absorption is exothermic and its solubility decreases with increasing temp. Due to this reason the process is carried out by cooling. ― Cooled HCl gas is admitted to bottom absorption tower which is connected with scrub up absorber and water is admitted into it from the top. Uses: metal surface cleaning, for manufacture of inorganic and organic chlorides, activating oil wells etc. HF acid manufacture ✔ It is the solution of HF gas in water. ✔ HF gas is made industrially by heating CaF2 with conc.H2SO4 then HF is purified by successive washing, cooling and fractional distillation to remove the SiO 2 present still with HF because it interfere with quality of the acid. SiO2 + 4HF → SiF4 +2H2O SiF4 + 2HF → H2SiF6 ✔ Absorption is similar as HCl but cu-alloy is used instead of fused silica as in HCl for absorption tower purpose. 5 Sulfuric acid(H SO ) 2 4 Properties of sulfuric acid ✔ Historical name: Oil of Vitriol ✔ Highly caustic strong mineral acid. ✔ Colorless to slightly yellow ✔ Highly corrosive ✔ Soluble in water at all concentrations ✔ Diprotic acid which may show different properties depending upon its concentration. 6 H2SO4 Manufacture ❖ Most widely used industrial chemical Involves 3 steps for its manufacture I. Burning of sulphur to SO2 II. Oxidation of SO2 to SO3 III. Absorption of SO3 in to water Two industrial manufacturing processes are : a) Contact process – heterogeneous catalytic process in which it uses solid V2O5 as a catalyzing agent to oxidize SO2 to SO3 b) Chamber process - homogeneous catalytic process in which it uses NO as a catalyst to oxidize SO2 into SO3. 7 Contact process Burning of S SO3 is difficult to dissolve in pure water since it is a highly exothermic reaction, which would produce a fine mist of sulfuric acid, difficult to condense; and could escape to pollute the air. Oxidation of SO2 Absorption 8 — Since the reaction is (exothermic) in oxidation of melted S and SO 2 , the heat obtained is used for melting of the sulphur. — If the concentration of the acid is less than 98%, it is sent to air dryer where it flows in the counter current direction to inlet air. The air takes away some heat of the acid and water. In that way H2SO4 is cooled and concentrated. 9 Chamber process • Uses impure H2SO4 (Nitrosoyl Sulphuric acid) containing oxides of nitrogen to dissolve SO2 in glover tower. • Hot SO2 in contact with impure H2SO4 in counter current direction , SO2 gets cooled and NOx’s (NO2 & NO) purged out. 2NO+O2→2NO2 SO2 + NO2 + H2O →H2SO4(impure dil.) + NO • Concentration and purification takes place in this part. • Some of the SO2 from lead chamber is absorbed in H2SO4 produced from glover tower in Gay-Lussac tower. 10 Lead-Chamber Process 11 Cont’d 12