MAGNESIUM HYDROXIDE LIME CAUSTIC SODA SODA ASH
Safety Primary compound (orderivatives) in "Milk of Magnesia", antacids, foodstuffs etc.. Comparatively safe to handle. Contact with eyes may cause temporary injury to the cornea. Contact with skin rarely causes irritation. Hazardous to handle. Contact with eyes can cause permanent loss of vision. Repealed and prolonged contact with skin may cause severe irritation, mild burns and, In extreme cases, systemic injuries due to absorption. Breathing dust or mist may cause intolerable discomfort to nose and throat. Extremely hazardous to handle. Contact with eyes can cause permanent loss of vision. Contact with skin may cause severe burns. Breathing vapor may cause damage lo the upper respiratory tract and the lungs. Moderately hazardous to handle. Contact with eyes may cause temporary injury lo the cornea, Contact will) skin may cause slight irritation. Breathing dust may cause painful irritation lo the nose and throat and prolonged exposure may cause systemic injury.
Environmental Effects As noted above, Is the base of "Milk of Magnesia" and, as such, is relatively harmless. Magnesium hydroxide is a natural mineral that poses no unusual threat to the environment. In fact, it is beneficial. Limestone (CaCO3) Is a naturally occurring mineral that poses no threat to the environment. Lime (CaO) and hydrated lime, however, are highly caustic and can cause immediate damage to the environment. Because caustic soda is highly corrosive, it can cause severe physical injury to plant and animal life if it escapes. A byproduct of neutralization is a sodium salt which, in high concentrations, may harm animals and vegetation.
Ease of Handling Supplied as a ready-to-use slurry or powder. Requires no special equipment except possibly an agitator in the slurry storage tank to prevent settling. Low temperatures create no special problems, because the slurry freezes at the same temperature as the water being treated, i.e. 32* F. Expensive solids-handling equipment is required. To maximize effectiveness, lime is often slaked into a hydrated slurry. A large storage hopper is needed. A slurry with the maximum 30% solids content will contain grit that causes rapid wear to costly valves and pumps. Requires elaborate safety equipment and rigorous, time-consuming safety proce­dures. Workers must be trained in safety and wear special clothing and goggles. Low temperatures can create major problems, because a 50% solution freezes at 57'F. Often requires temperature sensors and heaters throughout the distribution system. Very difficult lo handle. Requires special solids-handling equipment. If used In solution, usually requires heated pipes and valves, because it freezes at 80° F. Generates carbon dioxide, which may cause foaming and various process problems.
Sludge With a wide range of acids and metals, creates a sludge that is very dense, fast-selling and easily filtered and dewatered. Less sludge, less cost. With heavy metals and sulphuric acid, lime creates large quantities of calcium sulfate dihydrate, a sludge that settles very slowly, and is difficult to filter and dewater. If effluent contains heavy metals, it creates large quantities of gel-like slow-settling sludge that is difficult to filter and dewater. If effluent contains heavy metals, it creates large quantities of gel-like slow-settling sludge that is difficult to filter and dewater.
Equipment Costs Less than for caustic soda, lime, and soda ash even with an agitator required to prevent slurry settling in storage. May be used in powdered form (MgO,Mg(OH)2 with minor modifications to installed feed systems. Equipment costs higher than for magnesium hydroxide. Equipment maintenance also higher because lime is abrasive. Greater than for magnesium hydroxide because of need for heated system and safety equipment. Corrosive. Greater than for magnesium hydroxide because it requires either expensive solids handling equipment or heaters for handling a solution that freezes at 80° F.
Residence Time (Reaction Speed) Moderately fast acting to 95% of neutral. Slower above pH 6, resulting in more controllable process conditions and enhancing flocculant performance in solids settling. Fast acting to full neutralization, but can become coated with, for instance, calcium sulphate, leading to high usage rates. Extremely fast-acting with most acids. Difficult process control conditions and inefficient solids removal frequently occur when neutralization takes place very rapidly. Fast-acting to full neutralization with most acids. Difficult process control conditions and inefficient solids removal frequently occur when neutralization lakes place very rapidly.
Dissolved Solids Effluent (based on stoichiometric ratios) 1.31 tons per ton of HCL. 1.23 tons per ton of sulphuric acid. Generates only soluble salts. 1.52 tons per ton of HCL. 1.75 tons of insoluble salt per ton of sulphuric acid. (CaSO4 2H2O) 1.60 tons per ton of HCL. 1.45 tons per ton of sulphuric acid. 1.61 tons per ton of HCL. 1.45 tons per ton of sulphuric acid.
Maximum pH if Over Treated Usually no higher than pH 9, the limit set for most countries (i.e. United Sates Clean Water Act) Can reach a pH of 12. Can reach a pH of 14. Can reach a pH of 11.
Availability Available throughout the World in powder or slurry form (growing demand could limit supply in some countries). Readily available throughout the World. Readily available, but because caustic soda is co-produced with chlorine, the supply varies widely from surplus to shortage. Price varies accordingly. Readily available throughout the world but produced in only a few localities in limited number of countries.