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"Wet-Flo" Tank Cars and Tank Trucks When dissolving the NaCN in the "Wet-Flow" system, position the tank car or tank truck so that the hose connection can be made between the pipe headers and the storage tank piping and pump.

Meter into the storage tank the amount of water needed to make the desired concentration of sodium cyanide solution. Dissolving sodium cyanide is endothermic (absorbs heat), so the water can be preheated to 100 F (38 C) to speed the dissolving. See "Chemical Reactions," pages 4-5. However sodium cyanide solution in storage should be kept below 80 F (27 C) to prevent decomposition (See Fig. 4). Decomposition rates become excessive at temperatures above 120-125 F (4952 C) with sodium formate and ammonia being formed. Since this reaction is exothermic, provision must be made to remove the heat generated or, once started, the reaction may get out of control releasing large quantities of ammonia (See Temperature Indicators, Insulation, Heating and Cooling under EQUIPMENT on page 18.)

Unloading Procedure (See Figure 17)

Open valves A, B, C, G, H, I and the filter valves. Start the pump, then open the valve to the caustic supply. Check the system for leaks. Add sufficient caustic to give a final concentration of above 0.5% NaOH in the sodium cyanide solution. Pump the water continuously through the tank car or truck and storage tank for three hours. Then check the concentration by chemical analysis or specific gravity every half hour until it remains constant for at least two check analyses. This indicates that all sodium cyanide is dissolved. Do not pump out solution until it is completely dissolved, otherwise the lines may plug with briquettes and saturated solution. Stop the pump and close valves A, B, and C. Open valves E and D and pump the NaCN solution from the tank car or truck into the storage tank. Finally, open valve F to drain the top header.

EQUIPMENT

MATERIALS OF CONSTRUCTION

Carbon steel equipment is satisfactory for sodium cyanide solutions where velocities are not over 4-5 ft/ sec (1.2-1.5 m/sec). At higher velocity, 304 stainless steel is recommended because an erosion-corrosion effect occurs on steel. Welded, not threaded, steel piping should be used even at very low velocities. For pumps, valves, instruments, etc., 304 stainless is recommended. Gaskets of compressed white asbestos, with Buna S bonded filler are recommended for sodium cyanide solutions. Red rubber gaskets are satisfactory at ambient temperatures.

SOLUTION STORAGE TANK

For Wet-Flo systems, sodium cyanide solution is stored in a tank, typically fabricated from 5/16-in carbon steel plate. Top openings are recommended for a 6 to 8-in (15.2 to 20.3-cm) pipe vent, a 3 to 4-in (7.6 to 10.2-cm) fill and circulating line, level indicator, high-level alarm, provision for water and caustic additions and manway. A top 4-in nozzle for a physical break in the water line will prevent back-flow. Bottom openings must also be provided for a 4-in (10.2-cm) circulating line, temperature control-alarm or thermometer and optional heating/ cooling coils. The size of the storage tank depends upon shipment size and concentration of sodium cyanide solution required. For example, 65 000 pounds (29 500 kilograms) of sodium cyanide makes 19 000 gallons (72 000 liters) of 35% solution. A vertical tank 16 ft (4.9 m) in diameter by 20 ft (6.1 m) high has a 30 000 gallon (113 600 liter) capacity which is sufficient to allow some outage and/or solution inventory. The tank may be lined with neoprene to prevent build-up of iron content, if this is critical to the process. All pipe nozzles should be schedule 80 pipe.

If the storage tank is located inside a building, all tank openings must be sealed and the tank vent routed outside the building to a safe location. This will prevent the accumulation of dangerous levels of hydrogen cyanide inside the building. (See "Caustic Addition" below.)

PUMP

A stainless steel pump with a 4-in (10.2-cm) suction and a 3-in (7.6-cm) discharge is needed. It should be capable of delivering 350 gallons (1325 liters) per minute at an 80-ft (24.4-m) head. Small pumps will not provide sufficient agitation for tank car cyanide dissolving. Totally

enclosed fan-cooled motors are generally used in outside installations.

WATER METER & SUPPLY

A 3-in (7.6-cm) water meter with preset totalizer and an automatic cut-off should be installed in the storage tank water addition line. There should be a physical disconnection in the water supply line to prevent cyanide from backing up into the water system.

PIPE AND HOSES

Carbon steel is satisfactory where velocities are not over 4-5 ft/sec (1.2-1.5 m/sec.). At higher velocity an erosion-corrosion effect occurs on steel, and 304 stainless steel is recommended. Welded, not threaded, pipe should be used. For gaskets, see "Materials of Construction above. Wet-Flo circulation piping of 4-in (10 2 cm) is recommended with all valves. pumps. etc., 10cated inside the dike and minimum flanges outside the dike. Tank car hoses should be wire-reinforced rubber with end connections constructed as an integral part of the hose. Banded slip-on end connections are not recommended. Hose pressure rating should be 225 psi minimum with burst pressure (including end connections) at least twice the rated pressure. Circulation piping system, including hoses, should be inspected before each use to protect against failure and a major spill Hoses should be long enough to permit hook-up regardless of the direction the car comes into the siding Tank car connections are located on each end, but only on one side of the car.

DRAINAGE CONTROL

The storage tank should be diked and have a sealed concrete bottom. No dike drain should be installed since It might leak or be left open. The unloading area should be curbed and drainage control provided which will prevent spilled cyanide solution from draining into public water courses. (See "Pipe and Hoses" above) Specific spill control requirements depend on surroundings and local regulations. Impounded sodium cyanide can be chemically destroyed by alkali chlorination or treatment with KASTONE® peroxygen compound and formaldehyde' (see WASTE DISPOSAL).

FILTER

To obtain a clear solution, a filter should be used. It should be so located that it filters the solution between the tank car and the storage tank, and also, between the storage tank and process.

LEVEL INDICATOR AND LEVEL ALARM

A manometer-type liquid level indicator is preferred over a float-type instrument as it is easier to read and can be installed at a convenient location. It is recommended that a high-level alarm system, equipped to shut off the pump, be installed to prevent overflowing the storage tank.

Also a Du Pont product

TEMPERATURE INDICATORS, INSULATION, HEATING AND COOLING

Since sodium cyanide solution should be stored above the crystallization point (See Fig. 5), but below 80°F (27°C) where decomposition becomes significant (See Fig. 4), a temperature control system may be needed. Tank insulation can be fiberglass with a sheet aluminum cover if climate and solution concentration warrant. Temperature probes mounted on the sides of the tank at several locations may be needed. Low pressure steam should be used to avoid localized overheating. The same coils can be used for circulating cold water (or other coolant) when necessary.

CAUTION: Read the CHEMICAL REACTION section carefully and note the dangers at high temperatures.

CAUSTIC ADDITION

To minimize highly toxic HCN formation and prevent color formation in the stored sodium cyanide solution, sufficient caustic (sodium hydroxide) is added to give a pH 12. Caustic addition to provide a 0.5% solution (50 lb NaOH per 1,000 gallons water) is usually sufficient. A 50% caustic solution can be used during the summer, but a 25% solution is recommended for winter to avoid freezing. Additions can be made directly to the tank or through a connection on the suction side of the pump while the water is circulating. In any case, caustic should be added before circulation to dissolve sodium cyanide is started. Where process chemistry prevents caustic addition, HCN formation must be controlled by other means as discussed in "Chemical Reactions", p. 4-5.

FLO-BIN CONTAINER HANDLING

Bins can be readily handled by a fork lift truck with 3 600 pounds (1600 kg) capacity. They can be stacked two high when loaded and three high when empty. Trailer trucks, while limited to twelve loaded bins by weight regulations, can carry up to twenty empty bins.

WASTE DISPOSAL

The entire process in which the waste is generated should be reviewed for possible reclamation and reuse of sodium cyanide as well as its disposal. If reclamation is not feasible, ion exchange, reverse osmosis and dialysis may be useful, but destruction is usually easier and more economical.

Chlorination

CAUTION: CONCENTRATED HYPOCHLORITE SHOULD NOT BE MIXED WITH CONCENTRATED CYANIDE SOLUTIONS OR SOLID CYANIDE BECAUSE HIGHLY TOXIC CYANOGEN CHLORIDE GAS WILL BE RELEASED.

Chlorination of sodium cyanide waste solutions can be accomplished by treatment with highly diluted solutions of sodium hypochlorite, calcium hypochlorite or by generating hypochlorite from NaOH and Cl2 gas. The choice of hypochlorite source is an economic and safety decision.

Hypochlorite reacts with cyanide ions (CN-) to produce highly toxic cyanogen chloride which, at pH 10-11, hydrolyses promptly to form cyanate ions (CNO-). Since cyanogen chloride is a poisonous gas with little water solubility, the treatment process must be designed and operated to prevent cyanogen chloride fumes. Fumes are controlled by limiting the cyanide concentration to a few thousand parts per million.

Further chlorination to destroy cyanate, sometimes referred to as "complete" chlorination, can be accomplished with additional chlorine. After reaction at pH 10.5 for 10 minutes or more, the pH must be reduced to 7.5-9, preferably 8-8.5, and maintained at that pH until the reaction is complete. Completion of both reactions requires at least 2 hours and can use 8 or more parts of Cl2 per part of CN (vs. about 3 parts of Cl2 for oxidation to CNO-) while producing CO2 and N2 as reaction products. Chlorination is effective for cyanide destruction and can be automated for continuous systems. However, other waste stream components are often chlorinated, which increases chlorine consumption and may produce undesirable by-products in the effluent.

Peroxide Oxidation

CAUTION: CONCENTRATED KASTONE® PEROXYGEN COMPOUND OR HYDROGEN PEROXIDE SHOULD NOT BE MIXED WITH CONCENTRATED CYANIDE SOLUTIONS OR SOLID SODIUM CYANIDE BECAUSE HIGHLY TOXIC HCN OR AMMONIA GAS COULD BE RELEASED. ALSO, HIGH HEAT AND OXYGEN GAS RELEASE MAY CAUSE FOAMING OR ERUPTION AND SPLASH WORKERS. DILUTION PREVENTS THESE PROBLEMS.

Du Pont KASTONE® Peroxygen compound contains hydrogen peroxide and special stabilizers and additives for use in treating cyanide wastes. Depending on the composition of the waste, additions of formaldehyde may also be desirable. The waste liquor is adjusted to pH 11 (10.5-11.5), formaldehyde added if needed, and KASTONE® mixed with the solution. The solution must be agitated mechanically or with air. The reaction rate is dependent on temperature. Dilute wastes can be warmed to 100-130 F, (38-54 C) but temperature elevation is usually less economical than adding 10-20%

excess KASTONE® to shorten reaction time (which is normally about 1⁄2-1 hour at ambient temperature). Dilution and controlled addition rate may be needed when treating more concentrated wastes.

Advantages of the KASTONE® process vs. the chlorination process include:

• The reagents are far less corrosive.

• Investment is usually substantially lower.

• Operating costs are frequently lower.

• Cyanogen chloride and chlorinated by-products are not produced.

• More concentrated cyanide wastes can be treated safely.

• The hazards of handling chlorine are avoided. By-products from KASTONE® treatment are cyanate, ammonia, and glycolic acid amide. Cyanate does not revert to cyanide in surface waters or sewage treatment systems according to a U.S. Public Health Service study. Both the cyanate and glycolic acid amide are biodegradable. The cyanate can be readily hydrolyzed in acid solutions to ammonia.

Hydrolysis

Hydrolysis is sometimes a practical treatment for strong sodium cyanide solutions. Simply boiling a 10% sodium cyanide solution for about 8 hours should reduce the cyanide content to the parts per million range where chemical oxidation can be used more effectively. Provisions should be made, of course, to accommodate the

ammonia that will be generated. The most effective and widely used chemical methods to destroy cyanide are oxidizing it to cyanate (CNO-) with hypochlorite or hydrogen peroxide. Both methods are effective for oxidizing free cyanide, ions, or loosely bonded complexes. Metal Complexes

Strongly bonded metal cyanides, especially iron cyanides (ferro- and ferricyanide), are apt to be found in cyanide waste streams. These will not be detected by simple analytical procedures, such as titration with silver nitrate, that are normally used for measuring cyanide concentration. However, they will be included in the "total cyanide" analyses using acid distillation procedures. These complexes are not effectively destroyed by the commonly used waste processes. If regulations require removal of these generally stable complexes of low toxicity, other treatment methods such as precipitation to produce a solid waste may be required.

HANDLING SPILLS

Sodium cyanide spills should be cleaned up promptly to minimize exposure to people and the environment Shovel and sweep spilled material into a drum or suitable container. Keep spilled material dry. If raining. covering the spill will reduce the solution of sodium cyanide and reduce run-off. Decontamination of an area and destruction of cyanide for disposal can be accomplished with dilute hypochlorite solution. A small amount of caustic (5 lb/100 gallons water) will keep the pH of a sodium cyanide solution around 12 and minimize hydrogen cyanide formation. A little caustic solution can be used in a wet drum or puddle until disposal or decontamination is completed.