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Gas Cleaning System - Fluoride Removal
December 19, 2001
Materials of Construction
Sodium Silicate Systems
Fluorides are sometimes present as a contaminant in the off-gases coming from a metallurgical processes. Fluorine (F) may be present in either volatile or non-volatile forms. Volatile forms may include hydrogen fluoride (HF) and silicon tetrafluoride (SiF4). Non-volatile forms may include sodium fluoride (NaF) and aluminum fluoride (AlF3). The amount and form of fluoride contaminants in the off-gas will be dependent on the pyro-metallurgical process, the contaminants in the feed, operation of upstream hot gas cleaning system, etc.
The presence of fluorides is undesirable because it will attack any equipment made of glass or containing silica. This includes the glass fibres in fibreglass reinforced plastic (FRP) equipment, piping or ducting. The silica in acid resistant brick lining and ceramic packing is also subject to attack by fluorides. If fluorides are permitted to enter the converter, it will attack the silica in that catalyst carrier resulting in disintegration of the carrier.
In an acid plant, hydrogen fluoride levels in the range of 0.3 to 0.5 mg/Nm3 (0.27 to 0.56 ppm) are desirable to prevent excessive damage to acid resistant linings and catalyst carriers.
The mechanism of fluoride attack is the reaction of hydrogen fluoride (HF) with silicon dioxide (SiO2) to form silicon tetrafluoride (SiF4).
4 HF + SiO2 D SiF4 + 2 H2O
Silicon tetrafluoride is soluble in water and will react with additional hydrogen fluoride to form fluorsilicic acid.
2 HF + SiF4 D H2SiF6
In an acid plant, hydrogen fluoride levels less than 0.5 mg/Nm3 are desirable.
The detrimental effects of fluorides can be eliminated by the proper selection of materials of construction and the design of the gas cleaning system.
If fluorides are present in the gas, the proper materials of construction must be employed to prevent damage to equipment.
The first process step exposed to the metallurgical off-gases is the Quench or Humidification Tower. The tower is usually brick-lined with acid resistant brick to protect the carbon steel shell from the high temperatures and the corrosive nature of the circulating weak acid used to cool the gas. If fluorides are present in the gas and circulating weak acid, the exposed surface of the acid resistant brick would be quickly attacked. The solution is to add a layer of carbon brick which is resistant to fluoride attack.
Downstream of the Quench Tower, the materials of construction switches to FRP for the gas ducting and equipment. FRP is the ideal material because of its resistance to weak acid and its low cost.
FRP equipment for use in an acid plant is generally constructed with a corrosion liner which consists of a resin rich layer with a glass liner (or veil) which gives the corrosion layer strength and aids in controlling the thickness of the layer. When fluorides are present, the glass fibres in the corrosion layer will be readily attacked since they are close to the surface. Once attack begins on the glass fibres, the fluorides will work their way further into the structure of the FRP, weakening its structure. Eventually, leaks may occur or in the worse case, the equipment will fail under the exposed vacuum or pressure.
The solution is to utilize a non-glass synthetic veil in the corrosion layer of the FRP laminate. This provides a corrosion layer that contains no glass which can be subjected to attack by fluorides. The most popular synthetic veil is a polyester material known by the trade name Nexus® although other similar materials are available from other suppliers.
Hydrogen fluoride is very soluble in water and this fact can be used to control fluoride levels in the process gas. Fluorine scrubbers use this fact to scrub the fluoride containing gas in an packed tower with a re-circulated solution of weak HF. The gas enters the bottom of the fluorine scrubber and flows upwards against a flow of scrubbing solution flowing down through the packed section. Hydrogen fluoride will be absorbed into the scrubbing solution and exit the packed section in equilibrium with the incoming scrubbing solution. The scrubbing efficiency is thus dependent on the equilibrium conditions at the top of the scrubber.
The vapour pressure of hydrogen fluoride over aqueous solutions is shown in the following graph.
A solution containing 5 percent HF at 25oC has a equilibrium vapour pressure above the solution of 0.1 mmHg which corresponds to about 130 ppm HF in the gas. If the concentration of HF in the scrubbing solution is maintained at a lower concentration by purging and the addition of fresh water, equilibrium concentrations of HF approaching zero in the gas can be achieved. The problem with this approach is the large purge and make-up rates that may be required to achieve the desired concentration of HF in the gas exit the scrubber.
The effectiveness of a fluorine scrubber can be increased by lowering the equilibrium vapour pressure at the top of the tower. This can be achieved by allowing the hydrogen fluoride to react to form a compound whose equilibrium vapour pressure is less than that of pure hydrogen fluoride in solution. A substance that achieves the desired affect is sodium silicate or “water glass”.
The reaction of hydrogen fluoride with the silicon oxide in sodium silicate forms fluosilicic acid.
6 HF + SiO2 D H2SiF6 + 2 H2O
The vapour pressure of HF above a solution of fluosilicic acid is significantly less than above a solution of hydrofluoric acid. This allows HF to be reduced to the required levels at the outlet of the fluorine scrubber.
The addition of sodium silicate should be 1 to 1.2 times the stiochiometric amount. The recommended detection methods should be utilized to increase or decrease the addition of sodium silicate as required to ensure fluorides are being removed.
The presence and concentration of fluorides can be determined in a number of different ways. Analysis of the weak acid effluent is one of the easiest ways of measuring the quantity of fluorides present in the process.
The effectiveness of the gas cleaning system in removing fluorides can be determined qualitatively by observing the effects of fluoride attack on a glass rod that is inserted in the gas stream downstream of the gas cleaning system prior to the Drying Tower. The glass rod is held in the gas stream by a holder which supports the glass rod to prevent breakage. The glass rod is periodically removed and inspected to determined if it has been etched by fluorides. If fluoride attack is evident, the operation of the gas cleaning system must be checked. If fluoride carryover is evident, the acid resistant brick lining and ceramic packing in the drying tower should be inspected to determine if any damage has occurred.
As discussed previously, sodium silicate can be added to the scrubbing solution to react with the hydrogen fluoride in solution to form fluosilicic acid. The formation of fluosilicic acid improves the removal of hydrogen fluoride by reducing the equilibrium partial pressure of HF above the scrubbing solution.
Sodium silicate is available as a 25 to 40 wt% solution in water. It is a viscous liquid which is slippery to the touch. Sodium silicate solutions are available in a variety of containers and shipment sizes. For large consumption rates, it is available in tank truck sizes.
Prior to use, it is generally diluted by a factor of 4 to 5 before injection into the scrubbing solution. Concentrated sodium silicate cannot be used directly since it will react with the weak acid in the scrubbing solution to form a gel.
A typical set up for a sodium silicate addition system will consist of a storage tank, circulating/transfer pump(s), dilution tank with agitator and injection pump(s).
The storage tank should be sized to hold sufficient concentrated sodium silicate solution to ensure continued supply between deliveries. The size of the storage tank should also take into consideration the normal shipment size. Typically, the tank should be sized for 1.5 to 2 times the normal shipment size. A carbon steel storage tank can be used to hold sodium silicate solutions. Contact with aluminum, tin, zinc (including galvanized metals) and lead should be avoided as hydrogen gas may be produced.
Concentrated sodium silicate is diluted with water in the agitated dilution tank. This can be done on a batch basis or continuous basis depending on the consumption rate of the diluted solution and the size of the dilution tank. Continuous dilution can done by controlling the addition of water in ratio to the amount of concentrated sodium silicate being added to the dilution tank. Flow meters can be used to measure the flow and automatic valves used to control the flow in the proper proportions. The addition of concentrated sodium silicate is controlled by the level in the dilution tank.
The pumps to be used in the system will depend on the flow and head requirements. Standard chemical duty pumps can be used. Pump seals should be specified to prevent leakage of sodium silicate out to the environment. When concentrated sodium silicate is exposed to air, water will evaporate from the solution and the sodium silicate will solidify forming a glass-like solid which will ‘freeze’ a pump shaft. Seal water is sometime used to prevent leakage of sodium silicate along the pump shaft. The use of seal water should be monitored during prolong shutdowns to avoid dilution of the concentrated solution.