Acid Production

ACID PRODUCTION. The most widespread production is of nitric acid (HN0₃), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), hydrofluoric acid (HF), carbolic acid (C₆H₅OH), and acetic acid (C₂H₄O₂); in significantly smaller quantities, oxalic acid (C₂H₂O₄), picric acid [C₆H₂OH(NO₂)₃], and others are produced. The main mineral acids in most cases are manufactured in the same plants. HN0₃ is usually obtained by distilling a mixture of Chilean saltpeter and H₂SO₄ from iron vats into a water receiver and is purified by vacuum distillation. The most recommended method from a professional hygiene standpoint for obtaining nitric acid is the Valentin method, which ensures the least amount of harmful gases entering the room air. In the Norwegian method (distilling air through an electric arc), NO and NO₂ are formed, which are then captured in a water-sprayed coke tower and converted there into HN0₃. By the Haber method, N is first obtained by fractional distillation of liquid air, then at 220 atm pressure with elevated temperature (in the presence of a catalyst), N is converted into NH₃, then (also by catalysis) NH₃ is oxidized into HN0₃.-In the production of H₂SO₄, SO₂ is first obtained by roasting FeS in pyrite furnaces. Then by the chamber method, SO₂ is oxidized by nitric acid in the presence of water vapor into sulfuric acid; the N oxides formed in this process, in the presence of water vapor and air, are again converted into HN0₃, which oxidizes new portions of SO₂. The obtained H₂SO₄ is purified by diluting it with water and adding BaS, then concentrated (by evaporation) and distilled. In the more modern contact method, significantly less harmful gas (mainly SO₃) enters the room than in the chamber method. Production process: purified and dried SO₂ is converted into SO₃ by oxygen at 400° (in the presence of a catalyst); SO₃, combining with water, gives 96-98% H₂SO₄; upon further saturation with SO₃, fuming sulfuric acid is obtained (technically - "oleum"). Hydrochloric acid is obtained by heating NaCl with H₂SO₄; the evolved HCl is first passed through a water-sprayed coke tower, from which as a 2-5% HCl solution it passes through a system of condensation pots until saturation. To obtain hydrofluoric acid, hydrogen fluoride is obtained by heating H₂SO₄ with CaF₂, and it is captured with water. Carbolic acid (phenol) is obtained by extracting coal tar with alkali; the liquid is acidified and phenol is distilled off.-Acetic acid is obtained by dry distillation of wood, with the wood vinegar remaining in the last fraction being evaporated with lime; H₂SO₄ is added to the obtained liquid and it is distilled again; the obtained acetic acid is treated with permanganate to purify it from SO₂ and is redistilled to obtain "anhydrous" acetic acid. Picric acid is technically obtained by heating sulfophenolic acid with concentrated HN0₃. The main hazards in acid production: 1) danger of burns; 2) danger of poisoning from acid vapors and other gases. These hazards in the production of various acids occur with different intensity and have far from the same professional-hygienic significance. The dangers of skin and mucous membrane burns for the corresponding groups of workers in acid production can be arranged in the following order in descending order of acid action: "aqua regia" (a mixture of H₂SO₄ and HN0₃), fuming H₂SO₄ (oleum), H₂SO₄, HN0₃, HF, HCl, acetic, carbolic, oxalic acids.-The danger of burns must be taken especially seriously in non-mechanized production and first of all in manual handling of acid carboys, during manual pouring and unloading. However, even with significant mechanization of production processes, the possibility of burns is not excluded, both during individual moments of normal work (taking samples, etc.) and especially in case of any malfunctions in the equipment (sudden splashing of acid when opening a "stuck" valve, damage to pipes, etc.). The work of repair fitters is particularly dangerous (lack of mechanization, equipment malfunctions).-Measures to prevent burns. 1. Maximum mechanization of production. 2. Careful supervision of equipment tightness. 3. Rational storage of acids: a) use of vessels made of material not corroded by acid (for H₂SO₄-steel, for HCl-glass, for HF-lead, ebonite, for HN0₃-glass, for picric acid-glass); b) filling vessels not more than 90% of volume (danger of wall rupture); c) protecting acid vessels from temperature fluctuations; d) placing vessels in tanks with edges high enough to contain the vessel contents in case of damage; e) clear labels on each vessel, and if necessary-special coloring of walls. 4. Special devices (safe rotating carts, etc.) for transporting and pouring acids. 5. During fitter work on equipment-special precautions: preliminary inspection of valves, drying and ventilating pipes, vats, etc., when opening an acid line, thorough inspection of upstream sections, placing lead shields before flanges (to prevent the action of splashes), careful and gradual removal of bolts, etc. 6. Prohibition of open flame in the work area. 7. Supplying workers with rational special clothing. 8. Rational flooring-non-slip and with good drainage. 9. Regular washing of floors with a soda solution. 10. Sufficient lighting of the entire area and especially all equipment. 11. Placement in several points of each workshop of vessels with neutralizing solutions, easily accessible by means of a tap, hose with a clamp, etc. 12. Installation in each room not only a water tap but also a device that can immediately douse a worker drenched in acid with a large amount of water. 13. Warning signs in dangerous places.-First aid for burns from nitric, hydrochloric, acetic, oxalic, and picric acids. Wash with plenty of water and then with a saturated solution of NaHCO₃. For burns from H₂SO₄ and "oleum", before washing with water, the acid must be wiped off with a clean cloth and then proceed as above. For burns from hydrofluoric acid-washing with water and then with a diluted solution of NH₃. For phenol burns-first washing with water, then "Linimentum calcariae". Many acids and their compounds release vapors and gases that act caustically (HCl, SO₂, SO₃, H₂SO₄) or are poisonous (picric, carbolic acids) or both at the same time (HF, nitrogen oxides). The caustic effect of acid vapors and their anhydrides is acutely manifested as irritation of the mucous membranes of the eyes, upper and deep respiratory tract and is expressed depending on the concentration and effectiveness of the gas in various degrees: from tearing, runny nose and cough (under general working conditions) to destruction of the cornea and pulmonary edema (in emergency cases). With chronic exposure, usually associated with relatively weak gas concentrations, mild forms of irritation are observed; gradual habituation is also noted (especially in relation to SO₂). In relation to poisonous gases, habituation is not observed, and the danger increases with the duration of exposure. The poisonous effect of acid gases is especially dangerous when it does not manifest itself immediately, thus warning the worker of the danger, but appears only after some time (see also Nitrogen, oxides, Picric acid, Fluorine, Chlorine).-Preventive measures. 1. Sealing of equipment. 2. Mechanization of production. 3. Rational general and local ventilation. 4. In rooms where, despite the above measures, corrosive gases and especially nitrogen oxides enter the air-frequent spraying of soda solution into the air. 5. Wearing respirators with filters soaked in solutions neutralizing the given acid during work; to prevent drying, 25% glycerin is added to the solutions; respirators must be made of material that does not react with acid, or their surface is coated with a layer that does not react with acid. 6. Supplying workers with protective glasses in rubber frames. 7. Instructing workers on how to act in case of gas breakthrough (short breath, covering mouth and nose with a cloth in the absence of a respirator, etc.). 8.

The workplace must have a sufficient number of isolating gas masks (in case of emergencies), and all workers must be trained in their use; in case of a gas leak, all work to eliminate the accident must be done wearing these gas masks. 9. A special medical examination of workers at acid plants is necessary, both before employment and periodically, repeated. The danger of explosion exists even with non-explosive acids (HCl, H₂SO₄, HNO₃, etc.), if they come into prolonged contact with the metal they corrode: the hydrogen released in this process, combining with the oxygen of the air, can cause an explosion under the action of flame. Particular caution must be exercised in handling explosive acids (crystalline picric acid explodes with careless heating of even a small area of the mass, on impact and shock, on interaction with metals—formation of explosive picrates). Very strong self-heating can lead to violent splashing due to the boiling of water when alkali, water, or any water-containing acid is poured into sulfuric acid. When diluting sulfuric acid with water, water must not be poured into the acid. The acid must be added in small portions to water, constantly stirring the mixture. In case of fire in a workplace where acids are present, the fire should be extinguished with sand, not with water or fire extinguishers, to avoid the possible development of large quantities of harmful gases.