HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 81
at relatively high gas velocities and thus make better use of the particles' kinetic energy. They have been found to be moderately effective for contact plants having wet-purification systems in reducing stack plume opacities.
Table 6. Physical and Hazardous Properties of Phosphoric Acid Chemical Designations
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Synonyms: Orthophosphoric acid; Chemical Formula: H,PO,Observable Characteristics - Physical State (as shipped): Liquid; Color:
Colorless; Odor: Odorless
Physical and Chemical Properties - Physical State at 15 "C and 1 atrn. : Liquid; Molecular Weight: 98 .OO; Boiling Point at 1 atm. :
>
266,>
130,>
403; Freezing Point: Not pertinent; Critical Temperature: Not pertinent;Critical Pressure: Not pertinent; Specific Gravity: 1.892 at 25 "C (liquid);
Vapor (Gas) Speciflc Gravity: Not pertinent; Ratio of Specific Heats of Vapor (Gas): Not pertinent; Latent Heat of Vaporization: Not pertinent; Heat of Combustion: Not pertinent; Heat of Decomposition: Not pertinent Health Hazards Information - Recommended Personal Protective Equipment: Goggles or face shield; rubber gloves and protective clothing;
Symptoms Following Exposure: Burns on mouth and lips, sour acrid taste, severe gastrointestinal irritation, nausea, vomiting, bloody diarrhea, difficult swallowing, severe abdominal pains, thirst, acidemia, difficult breathing, convulsion, collapse, shock, death; General Treatment for Exposure:
INGESTION: do NOT induce vomiting; give water, milk, or vegetable oil.
SKIN OR CONTACT: flush with water for at least 15 min.; Toxicity by Inhalation (rnreshold Limit Value) : 1 .O mg/m3; Short-Term Inhalation Limits:
Not pertinent; Toxicity by Ingestion: Grade 3, LD,, 50 to 500 mg/kg; Late Toxicity: None; Vapor (Gas) Irritant Characteristics: Not volatile; Liquid or Solid Irritant Characteristics: Fairly severe skin irritant; may cause pain and second-degree burns after a few minutes contact; Odor Threshold: Not pertinent.
Phosphorus is always shipped and stored under water to prevent combustion. The tank car of phosphorus is heated by steam coils to melt the water-covered
phosphorus. Heated water at about 135 O F is then pumped into the tank car and displaces the phosphorus, which flows into a storage tank. A similar system using hot displacement water is frequently used to feed phosphorus to the burning tower.
A number of air contaminants, such as phosphine, phosphorus pentoxide, hydrogen sulfide, and phosphoric acid mist, may be released by the phosphoric acid process. Phosphine (PH,) is a very toxic gas, and may be formed by the
HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 83
hydrolysis of metallic phosphides that exist as impurities in the phosphorus.
When the tank car is opened, the phosphine usually ignites spontaneously but only momentarily. Phosphorus pentoxide (P40,0), created when phosphorus is burned with excess air, forms an extremely dense fume. The fumes are submicron in size and are 100 percent opaque.
Except for military use, phosphorus pentoxide is never released to the atmosphere unless phosphorus is accidentally spilled and exposed to air. Since handling elemental phosphorus is extremely hazardous, stringent safety
precautions are mandatory, and phosphorus spills are very infrequent. Hydrogen sulfide (H2S) is released from the acid during treatment with NaHS to precipitate sulfides of antimony and arsenic and other heavy metals.
Removal of these heavy metals is necessary for manufacture of food grade acid.
H2S is highly toxic and flammable. The odor threshold is less 0.2 ppm and has the characteristic odor of rotten eggs. Refer to Table 7. In practice, however, H,S is blown from the treating tank and piped to the phosphorous-burning tower where it is burned to SO2. Evolution of H2S is also minimized by restricting the amount of NaHS in excess of that needed to precipitate arsenic and antimony and other heavy metals.
The manufacture of phosphoric acid cannot be accomplished in a practical way by burning phosphorus and bubbling the resultant products through either water or dilute phosphoric acid. When water vapor comes into contact with a gas stream that contains a volatile anhydride, such as phosphorus pentoxide, an acid mist consisting of liquid particles of various sizes is formed almost instantly.
The particle size of the phosphoric acid aerosol is small, about 2 microns or less, and it has a median diameter of 1.6 microns, with a range of 0.4 to 2.6 microns.
The tail gas discharged from the phosphoric acid plant is saturated with water vapor and produces a 100 percent opaque plume. The concentration of phosphoric acid in this plume may be kept small with a well-designed plant. This loss amounts to 0.2 percent or less of the phosphorus charged to the combustion chamber as phosphorus pentoxide. All the reactions involved take place in closed vessels. The phosphorus-burning chamber and the hydrator vessel are kept under a slight negative pressure by the fan that handles the effluent gases. This is necessary to prevent loss of product as well as to prevent air pollution. The hydrogen sulfide generated during the acid purification treatment must be captured and collected, and sufficient ventilation must be provided to prevent an explosive concentration, for hydrogen sulfide has a lower explosive limit of 4.3 percent. The sulfiding agent must be carefully metered into the acid to prevent excessively rapid evolution of hydrogen sulfide, The hydrogen sulfide can be removed by chemical absorption or by combustion. Weak solutions of caustic
soda or soda ash sprayed countercurrently to the gas stream react with the
hydrogen sulfide and neutralize it. The hydrogen sulfide may also be oxidized in a suitable afterburner. The phosphoric acid mist in the tail gas is commonly removed by an electrostatic precipitator, a venturi scrubber, or a fiber mist eliminator. All are very effective in this service.
Table 7. Physical and Hazardous Properties of Hydrogen Sulfide Chemical Designations
-
Synonyms: Sulfuretted hydrogen; Chemical Formula: H,SObservable Characteristics
-
Physical State (as shipped): Liquid under pressure; Color: Colorless; Odor: Offensive odor, like rotten eggsPhysical and Chemical Properties
-
Physical State at 15 "C and I atrn. : Gas; Molecular Weight: 34.08; Boiling Point at 1 atm. : -76.7 O F , -60.4 "C, 212.8 O K ; Freezing Point: -1 17, -82.8, 190.4; Critical Temperature: 212.7,100.4, 373.6; Critical Pressure: 1300 psia; Spec@c Gravity: 0.916 at -60 "C (liquid); Vapor (Gas) Specific Gravity: 1.2; Ratio of Specific Heats of Vapor
(Gas): 1.322; Latent Heat of Vaporization: 234 Butflb; Heat of Combustion: -
6552, -3640, -152.4; Heat of Decomposition: Not pertinent Health Hazards Information
-
Recommended Personal Protective Equipment: Rubber-framed goggles; approved respiratory protection;Symptom Following Exposure: Irritation of eyes, nose and throat. If high concentrations are inhaled, hyperpnea and respiratory paralysis may occur.
Very high concentrations may produce pulmonary edema; General Treatment for Exposure: INHALATION: remove victim from exposure; if breathing has stopped, give artificial respiration; administer oxygen if needed. EYES: wash with plenty of water; Toxicity by Inhalation (77zreshold Limit Valrre): 10 pprn;
Short-Term Inhalation Limits: 200 ppm for 10 min.; 100 ppm for 30 min. and 50 ppm for 60 min.; Toxicity by Ingestion: Hydrogen sulfide is present as a gas at room temperature, so ingestion not likely; Lnte Toxicity: Data not available; Vapor (Gas) Irritant Characteristics: Vapor is moderately irritating such that personnel will not usually tolerate moderate or high vapor
concentrations; Liquid or Solid Irritant Characteristics: Minimum hazard. If spilled on clothing and allowed to remain, may cause smarting and reddening of skin; Odor mreshold: 0.0047 ppm.
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HAZARDS IN THE CHEMICAL PROCESS INDUSTRIES 85