MXPA99000649A - Elimination of oxide layers containing fluoride using an alumi salt solution - Google Patents

Abstract

The present invention relates to: fluoride-containing oxide layer can be removed from metal surfaces such as for example titanium, titanium alloys, nickel alloys and stainless steel by contacting the metal surfaces with an aqueous salt solution of an inorganic acid, including your hydrates. The cationic portion of the salt may be aluminum, iron and mixtures of these. The anionic portion of the salt may be a chloride, a nitrate, a sulfate and mixtures thereof. The contact occurs in the absence of the addition of an acid, such as, for example, hydrochloric, nitric or sulfuric acid. The presence of the aqueous salt solution with the dissolved fluoride oxide layer does not accelerate or increase the normal rate of metallic corrosion that can occur in the absence of the aqueous salt solution or any cleaning agent.

Description

ELIMINATION OF OXIDE LAYERS OUE CONTAINS FLUORIDE USING AN ALUMINUM SALT SOLUTION This application claims the benefit of United States Provisional Application Number 60 / 021,889, filed July 17, 1996. BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the removal of oxide layers from metal surfaces, and more particularly, to the removal of fluoride-containing oxide layers from metal surfaces. 2. Description of the Prior Technique When the carbon other organic materials containing ash are gasified in a cooling gasification system with partial oxidation at high temperature and high pressure, the material of the ash is usually divided between coarse slag, finely divided slag particles and water-soluble ash components. The water is used in the system to convert the feed carbide into slurry, to cool the hot synthesis gas, also referred to as "syngas" and to cool the secondary product of the hot slag. Water is also used to purify the particulate matter from the syngas, and to help transport the secondary product of the slag out of the gasifier. The fluoride oxide layer of calcium and fluoride Magnesium that is formed in the evaporator tubes is usually chemically removed by inorganic acids such as, for example, sulfuric, hydrochloric or nitric acids. When sulfuric acid is used for the removal of the oxide layer, CaS04 sometimes precipitates. During the acid cleaning of the fluoride oxide layer, a corrosive hydrofluoric acid forms in the cleaning solution and certain metals and metal alloys, such as titanium, nickel and stainless steel, may be subject to severe corrosion of hydrofluoric acid. The presence of the fluoride ion (F) in the solution interferes with the protective oxide films that are formed on these metals and allows the dissolution of the titanium, iron and nickel ions in an acid solution. Therefore, chemical cleaning of the fluoride oxide layer by the use of acids only in the process equipment is not practical. It has also been observed that the calcium oxide layer can be chemically removed by the use of tetra-acetic acid of ethylene diamine.

The oxide layer can also be removed by mechanical means such as scraping or impact with a hammer or by abrasive pressure water blasting. However, chemical cleaning is preferred and is generally more complete because the coating Rust can dissolve and be removed in places where the nozzle of the pressurized water jet can not reach. Therefore, it is desirable to dissolve chemically the fluoride oxide layer of the equipment constructed of titanium or stainless steel. Titanium and stainless steel are commonly used in the wastewater treatment industry, especially in the construction of wastewater evaporators. The literature has also focused on the problem of the corrosion of hydrofluoric acid in the processing equipment made of stainless steel, nickel alloys and titanium alloys. Koch, G.H. , "Localized Corrosion in Halides That Are Not Chlorides", Environmental Effects. June 1993, reveals that ferric or aluminum ions can inhibit corrosion. The effect of water solutions and their corrosivity in purifiers of the process of desulfurization of the combustion gas has also been studied. These solutions contain chlorides, fluorides and sulfates with low pH, for example, 800 mg / kg fluoride at a pH of 1. The addition of flying ash minerals containing significant amounts of silicon, iron and aluminum can inhibit titanium corrosion in other solutions that contain aggressive fluoride. It was also discovered that if 10,000 mg of aluminum / kg (added as aluminum sulfate) were added to a corrosive acid solution containing 10,000 mg / kg of chloride and 1,000 mg / kg of fluoride, the solution is no longer corrosive to titanium. COMPENDIUM OF THE INVENTION The fluoride-containing oxide layer can be removed from metal surfaces such as for example titanium, titanium alloys, nickel alloys and stainless steel by contacting the metal surfaces with an aqueous salt solution of an inorganic acid, including its hydrates. The cationic portion of the salt can be aluminum, iron and mixtures thereof. The anionic portion of the salt may be a chloride, a nitrate, a sulfate, and mixtures thereof. The contact occurs in the absence of the addition of an acid, such as, for example, hydrochloric, nitric or sulfuric acid. The presence of the aqueous salt solution with the dissolved fluoride oxide layer does not accelerate or increase the normal rate of metallic corrosion that can occur in the absence of the aqueous salt solution or any acidic cleaning agent. DESCRIPTION OF THE REFERRED FORMS OF REMOVAL In order to conserve water, the operating units of the gasification system seek to recirculate the process water, usually after a purification treatment, such as finely removing particulate slag. divided or "fine slag" in a solids sedimentation tank. As the gasification reaction consumes water through the production of hydrogen in the synthesis gas, there is usually no need to remove the water from the system to prevent its accumulation. TO Despite this, a portion of the waste water from the process, also referred to as the aqueous effluent, gray water, or purge water, is usually removed from the system as a purge wastewater stream to avoid an excessive buildup of salts corrosive, in particular chloride salts. As shown in Table 1, presented below, with the gasification data of eastern coal from the United States with a high chloride content, the composition of the waste water purge of the gasification system is quite complex. For raw food materials with relatively high levels of chloride, the main component of wastewater is ammonium chloride. TABLE 1 CONTENT OF EASTERN CARBON ASH WITH HIGH CHLORIDE CONTENT Some materials found in the ash are partially soluble in water, that is, a portion of the material remains in the solid slag or fines of the ash and a portion dissolves in the water. For example, sodium and potassium compounds dissolve in water just like their ions, and they belong in solids like sodium minerals. The boron compounds dissolve in water as boric acid and borate ions and remain in solids as oxidized boron minerals. The aluminum, silicon, calcium and magnesium compounds are mainly insoluble, and the fluoride compounds are also mainly insoluble. As the purge of the waste water from the gasification system contains potentially harmful salts and other components for the environment, treatment is necessary before the water can be discharged. The treatment for wastewater for a variety of contaminants can be Some form to be elaborated and cost, therefore, other more economical means to treat the waste water are desirable.

The distillation of wastewater or saline under certain conditions is an effective and economical means of recovering relatively pure water from wastewater. Suitable means for distilling the waste water by gasification include the evaporation of the falling film and evaporation by forced circulation. This invention offers a means to remove the fluoride oxide layer that forms on the metal surfaces of these evaporators, and in any other equipment. In evaporation with falling film, the heat exchanger of the main system is vertical. The saline solution to be evaporated is introduced into the upper part of the tubes of the heat exchanger and removed at the bottom. The saline solution is pumped to the top of the tubes from a reservoir of saline solution located below the tubes of the heat exchanger. The saline solution falls down through the tubes like a film on the inside walls of the tube, receiving heat in such a way that the water contained here evaporates and forms a vapor as the saline solution descends. A mixture of saline solution and steam exits through the lower part of the tubes of the heat exchanger and enters the reservoir of the saline solution, where the water vapor and the solution are separated. concentrated liquid saline. The vapor from the top of the saline solution tank, and the residual concentrated liquid saline solution is collected in the saline tank where it is recirculated by a pump to the top of the heat exchanger tubes. The steam can then be condensed to form a water distillate that can be recycled to the gasification system. Feeding water, such as waste water from the effluent from the gasification system, can be added continuously to the deposit of the saline solution, a portion of the concentrated salt solution is continuously removed for crystallization and recovery of the concentrated salts contained in it. In an evaporation by forced circulation, the heat exchanger of the main system is horizontal, and the liquid saline solution is pumped through the tubes and the steam is introduced on the side of the exchanger cover to heat the salt solution. The saline solution does not boil as it travels through the tubes because there is enough pressure inside to avoid boiling. The hot saline solution that exits the exchanger tubes then rises to a reservoir of saline solution located above the heat exchanger. As the saline travels upward, the pressure drops and the hot saline boils to form a mixture of two phases of concentrated saline solution and water vapor. When the two-phase mixture enters the reservoir of the saline, the water vapor is separated from the saline solution, and leaves the reservoir to a condenser where the water vapor condenses to form distilled water. The saline solution is recycled to the evaporator by means of a recirculation pump, eliminating a portion such as the purge stream from the saline solution for its crystallization and subsequent salt recovery. As with the falling film evaporator, the feed water is added to the saline solution reservoir or to the saline recirculation line, although both the falling film and forced circulation evaporators are Commonly used for water distillation applications, its use depends on the rate of formation of the oxide layer and its accumulation on the surfaces of the evaporator heat exchanger.The removal of the oxide layer from the surfaces of the heat exchanger The evaporator and the tank are very important because the formation of the oxide layer on the surfaces of the equipment acts as an insulator and must be removed periodically in order to operate the evaporator unit effectively. Table 2, below, was formed from evaporation of gray water of gasification where the film evaporator was used in calda and in forced circulation in series. The components of the main oxide layer are silicon (Si02), calcium fluoride (CaF2) and magnesium fluoride (MgF2). TABLE 2 COMPOSITION OF THE OXIDE LAYER OF THE TUBE AND THE LAYER OF OXIDE OF THE DEPOSIT FROM THE EVAPORATION OF PURGE WATER In accordance with the present invention, the fluoride oxide layer can be removed from titanium, titanium alloys, nickel alloys and stainless steel by using an aqueous salt solution of an inorganic acid, including its hydrates. The cationic portion of the eal can be aluminum, iron or mixtures thereof. The anionic portion of the salt can be a chloride, a nitrate, a sulfate, and mixtures of these. Contact occurs in the absence of the addition of an acid, such as hydrochloric, nitric or sulfuric acid. The presence of the aqueous salt solution with the dissolved fluoride oxide layer does not accelerate or increase the normal speed of metal corrosion that can occur in the absence of an aqueous salt solution or any acidic cleaning agent.

Preferred salts are aluminum salt solutions made from aluminum chloride, aluminum sulfate, aluminum nitrate and their hydrates, and mixtures thereof. Aluminum nitrate is the preferred aluminum salt when the equipment being treated is part of a partial oxidation gasification system, because the occupied solution can return to the gasification system, and has less impact on the gasifier supply . The nitrate components of the aluminum nitrate salt can become part of the synthesis gas, such as N, NH3 or CO. In contrast, aluminum chloride adds chloride to the feed in the form of ammonium chloride, and aluminum sulfate adds sulfur and calcium sulfate is precipitated in the evaporator. Although iron salts of inorganic acids can also be used to dissolve fluoride dioxide layers, iron salts are generally not as effective as aluminum salts on a base of molar comparison to dissolve the fluoride oxide layer and inhibit the corrosion of titanium fluoride in acid solutions. The aqueous salt solution of the inorganic acid should have a concentration of about 1% to about 40%, preferably about 15% to about 20% and a temperature of about 32 ° F to about 212 ° F. The salt solution is more effective in dissolving fluoride oxide layers with respect to the rate and amount dissolved if the solution is heated to a temperature of about 100 ° F to about 212 ° F, and preferably about 175 ° F to approximately 212 ° F. In a comparison test, the oxide layer that dissolved in 90 minutes at 100 ° F could dissolve in one minute at 175 ° F. The aqueous inorganic salt solution comes into contact with the surface of the oxide layer for a sufficient time to carry out the elimination or dissolution of the chloride oxide layer, which is usually from about 30 minutes to about 24 hours , and preferably from about 1 hour to about 3 hours. A combination of inorganic salt solutions, including solutions of their hydrates, can also be used. The initial pH of the aqueous salt solution is generally at least about 1.5. Before or after surface treatment With the solution of aqueous aluminum salt of inorganic acid, a solution of an alkali metal hydroxide such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) can be used to contact and treat the metal surface for the purpose of to remove any oxide layer that contains silicon, or any layer of iron cyanide dioxide. The treatment with alkali metal hydroxide, particularly the NaOH treatment, is generally selected as the first solution for the cleaning of the oxide layer, mainly because the caustic solution is less expensive than the aluminum salt solution, in particular the aluminum nitrate solution. The alkali metal hydroxide solution should have a concentration of about 1% to about 25%, and preferably 2% to about 6%, and should be heated to a temperature of about 170 ° F to about 212 ° F, or to the point of boiling the solution at atmospheric pressure. The alkali metal hydroxide solution should be contacted with the surface of the oxide layer for a sufficient time to effect the removal of the silicon oxide or iron cyanide layer, which is usually from about 30 minutes to about 24 hours, and preferably from about 2 hours to about 6 hours. A hydroxide mixture can also be used sodium and potassium hydroxide »A sodium nitrate inhibitor is generally used with caustic when the oxide layer of titanium is removed. After finishing the caustic cleaning operation, the caustic solution must be removed from the equipment, for example by draining it, before introducing the aqueous inorganic salt solution, and vice versa. No special cleaning is necessary after the removal of each cleaning solution. In this way, the following cleaning solution, i.e. aqueous inorganic salt solution, can be introduced into the equipment and removed in a similar manner. The combined neutralized solutions employed of the sodium hydroxide and the aqueous inorganic salt solution can be combined, diluted with water to a concentration of about 95% water and neutralized to a pH of about 7 using additional sodium hydroxide, if necessary. The neutralized occupied cleaning solution can be subsequently used to form a slurry of the feedstock, such as carbon, for a partial oxidation reaction. In this way, for example, the components of fluoride, eodium, aluminum and silicon become components of the slag of the secondary product. If the occupied alkaline solution is recycled to the gasifier, the recycled solution should be added in small amounts to the feedstock in such a way that the feed concentrations of sodium or potassium are not increased significantly because they can cause an adverse effect on the refractory lining of the gasifier. An unneutralized occupied aluminum salt solution can be recycled to the feed of the gasifier as long as it is mixed with the feedstock in a sufficiently low percentage so that the pH of the feedstock is not reduced below 6.0 It is observed that by using the aqueous salt solution without an acid, instead of using an inorganic acid cleaning solution with an aluminum salt added, the cleaning process does not accelerate corrosion or increase the speed of corrosion, while with an acid, care must be taken to add enough aluminum inhibitor to reduce or prevent the acceleration of corrosion. As the amount of oxide layer in the equipment is not exactly known before cleaning and there is an economic need to save chemical cleaning solutions, this is an important consideration. The means to determine if additional cleaning solution needs to be added to the equipment can be determined by an analysis of total dissolved solids in which a filtered cleaning solution is taken from the equipment to be treated and dried at 105 ° C and measured the weight of the residue.

The concentration of total disulfide solids of the initial cleaning solution and the cleaning solution in contact with the oxide layer can be used to determine whether the cleaning solution is saturated with the oxide layer compounds. A molar ratio of 0.5 of silicon with alkaline hydroxide and a molar ratio of 0.65 of calcium fluoride with aluminum salt solution should be used to determine the saturation point of the cleaning solution. In this way, the amount of cleaning solution used can be reduced. In the examples, and throughout the specification, all concentrations are in percent by weight, unless otherwise specified. EXAMPLES 1-6 The purge water of the composition in Table 1 is evaporated in an evaporator with falling film to produce a mixture of water vapor and saline. This mixture is fed to the salt solution tank of an evaporator with a film in which the water vapor is separated from the saline solution and fed to a condenser to recover the distillate from the water. days, a layer of titanium surface oxide develops inside the evaporator tubes and on the surface of the high nickel alloy Hastelloy ™ C-276 (Haynes Metals Co.) that forms the deposit.

The oxide layer is mechanically removed from the metal surface of the salt solution tank by flaking off flakes from the surface of the evaporator tubes by impacting the outside of the titanium tubes with a hammer. The composition of the oxide layer is about 50% amorphous silicon and 50% calcium fluoride. Separated samples of 6 grams of the oxide layer are initially contacted with 100 grams of a sodium hydroxide solution at a concentration of 6% d 10% at a temperature of 170 ° F for at least 2 hours. After the treatment period, the caustic solution is analyzed by means of the Inductively Coupled Plaema Instrument (ICP) Method for metals and ion chromatography for fluoride, and the weight of Si, Ca and F dissolved by the solution is determined. caustic The oxide layer sample is then contacted with an aluminum nitrate solution (11.2%, 12% or 16%) at a pH of 1-2 and a temperature of 100 ° F or 170 ° F for minus 2 hours In EXAMPLES 4-6, the aluminum nitrate solution also contains 0.5 or 1% sodium nitrate (NaN03) which is used to inhibit the formation of the hydride phase in the titanium. After the treatment period, the aluminum nitrate solution is analyzed by the ICP Methods for metal and ion chromatography for the chloride, and the weight of the Si, Ca and F is determined. dissolved by the aluminum nitrate solution. The examples show that a fluoride containing a layer of oxide is effectively removed using solutions of aluminum nitrate, with more than 90% removal of the oxide layer achieved in Examples 1, 4 and 6. The results are recorded in Table 3, below. TABLE 3 ELIMINATION OF THE OXIDE LAYER FROM THE EVAPORATOR TANK WITH FILM IN FALL NOTE: The maximum capacity of the NaOH solution is to dissolve 0.5 moles of Si for each mole of NaOH (2 moles of NaOH are required to form 1 mole of sodium silicate). The solution is used completely when the ratio of S to NaOH is 0.5. The maximum capacity of the A1 solution. { N03) 3 at 100 ° F is to dissolve approximately 1.3 moles of fluoride (0.65 moles of CaF2) for each mole of aluminum (previously determined in the dissolution tests of caF2). The solution is used completely when the ratio of fluoride to aluminum is 1.3 or the ratio of fluoride to NO- is 0.43. TO 174 ° F, 1.6 moles of fluoride (0.8 moles of CaF2) are dissolved per mole of aluminum. TABLE 3 (Continuation) ELIMINATION OF THE OXIDE LAYER FROM THE EVAPORATOR TANK WITH FILM IN FALL fifteen The residue of Step 2 was subjected to further successive cleanings using fresh solutions of AlfNO-,), and NaOH until the entire oxide layer was completely dissolved. The following results were obtained and presented in order of succession with the concentration of the solution, time, temperature and residuals in percentage after cleaning. Third cleaning - 11.2% at £ N03) 3-3 hours-14%; Fourth Cleaning - 11.2% Al (N03) 3-6 hours-13%; fifth Cleaning - 2% NaOH - 2 hours - 6%; Sixth Cleaning - 6% NaOH - 1.5 hours completely dissolved the oxide layer. The residue of Example 3 was subjected to 3.2 g of 10% NaOH-1% of NaNOj at 170 ° F for 5.5 hours and the residue was reduced to 12% (the main component of this residue was CaF2). X-ray diffraction analysis showed that this residue predominantly contains A12 (0H) 3F3. EXAMPLE 9 Two aqueous solutions, designated "A" "B" are prepared with a content of 1% fluoride from calcium fluoride powder, and 4% aluminum chloride added as a corrosion inhibitor. A concentration of 1% hydrochloric acid is also added to the solution. Both solutions are heated to 100 ° F and contacted with titanium grade 2 for 24 hours. The rates of corrosion and other data are recorded in Table 4. TABLE 4 An acceptable corrosion rate will be less than about 10 mils / year, and preferably less than about 5 ils / year. The corrosion rate of solution A is very high and result in a substantial loss of the metal. It is clear that the use of a solution acid to dissolve the fluoride oxide layer, even with a corrosion inhibitor, can result in disastrous corrosion when the fluoride oxide layer of the titanium is cleaned using an acid. The problem with using an acid cleaner is that the amount of the fluoride oxide layer in the unit is not known ahead of time. Therefore, the amount of aluminum corrosion inhibitor would have to be extremely above the dose as a precautionary measure. By using an aluminum salt solution without an acid, the fluoride oxide layer is dissolved and the corrosion rates of the titanium are acceptably low.

Claims (10)

1. CLAIMS 1. A process for removing the fluoride-containing oxide layer from a metal surface that includes contacting the metal surface with a sufficient amount of an aqueous salt solution of an inorganic acid, including its hydrates, to dissolve the coating. fluoride-containing oxide, wherein the cationic portion of the salt is selected from the group consisting of aluminum, iron, and mixtures thereof, and wherein the anionic portion of the salt is selected from the group consisting of the chloride, nitrate , sulfate and mixtures of these, and where the mentioned contact occurs in the absence of the addition of an acid.
2. 2. The process of claim 1, wherein the contact of the aqueous salt solution with the metal surface and its presence with the dissolved fluoride oxide layer does not increase the normal corrosion rate of the metal mentioned which can occur in the absence of the Solution of aqueous salt or any acid cleaning agent.
3. 3. The process of claim 1, wherein the aqueous salt solution includes at least one aluminum salt selected from the group consisting of aluminum nitrate, aluminum sulfate, and aluminum chloride. .
4. The process of claim 1, wherein the initial pH of the aqueous salt solution is at least 1.5.
5. 5. The process of claim 1, wherein the The salt concentration of the inorganic acid is from about 1% to about 40%.
6. The process of claim 1, wherein the metal surfaces include the heat exchanger tubes of the evaporator having layers of oxide deposited therein by contact with the purge of the wastewater from a partial oxidation gasification plant. .
7. 7. The process of claim 1, wherein the metal surfaces are selected from the group consisting of titanium, titanium alloys, nickel alloys and stainless steel.
8. 8. The process of claim 3, wherein an alkali metal hydroxide solution is contacted with the metal surface before or after contacting the aqueous solution of the aluminum salt or the hydrate of the aluminum salt.
9. 9. The process of claim 8, wherein the concentration of the alkali metal hydroxide solution ranges from about 1% to about 25%.
10. 10. The process of claim 8, wherein upon completion of the contact operation, an occupied solution of the alkali metal hydroxide is formed and an occupied solution of the aluminum salt of an inorganic acid or hydrate is formed, and the occupied solution of the alkali metal hydroxide and the occupied solution of the aluminum salt of an inorganic acid or hydrate are combined and fed to a gasifier in a gasification system with partial oxidation.