There is several methods for removing calcium sulfate scale deposits from surfaces.
1, Strong concentrations of alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, shall be used to effect prolonged contact with the scale deposits so that the latter can be converted to calcium hydroxide The caustic solution must be continuously circulated over the scale encrusted surface and the reaction is frequently not complete because of the occluding nature of the hydroxide conversion product. This secondary insoluble calcium hydroxide product must be subsequently contacted with hydrochloric acid in order to produce water soluble calcium chloride which may then be removed by dissolving it in an aqueous solution.
2, Calcium sulfate scale deposits have also been removed by utilizing ammonium bicarbonate. However, as with strong caustic solutions, the ammonium bicarbonate converts the sulfate scale into another insoluble product, calcium carbonate. The latter must subsequently be contacted with an acid, such as hydrochloric acid, before it can be removed by dissolving in water. The rate at which the ammonium bicarbonate converts the calcium sulfate scale decreases in ratio to the amount of conversion product because of the occlusion which occurs on the scale surface. Accordingly, removal of the scale deposits is frequently incomplete and the efficiency of this method cannot be appreciably increased even if the ammonium bicarbonate solution is continually recalculated over the scale encrusted surface.
3, Another method presently known in the prior art for removing calcium sulfate scale deposits involves the use of solutions of alkaline chelating or sequestering agents such as ethylenediamine tetraacetic acid (EDTA), its tetrasodium salt, and nitrilo triacetic acid trisodium salt. These solutions are maintained at substantially higher than ambient temperature and must remain in contact with the scale for extended periods of time. However, the chelation or dissolution of the scale is generally slow.
The removal of calcium sulfate scale deposits by utilizing only a chelating agent, such as EDTA (ethylenediamine tetraacetic acid), is normally a rather slow procedure since the EDTA attacks the deposits very slowly. However, the rate of this reaction can be greatly increased by combining the EDTA with a carbonate, such as sodium bicarbonate, thereby increasing the rate of calcium sulfate dissolution to nearly twice that with EDTA alone. The mechanism of this reaction can be described as follows:
The carbon dioxide released by this reaction causes the scale deposits to break apart and thereby expose additional deposit surfaces which are more readily attacked by the EDTA.
It has now been discovered that the addition of particular surfactants i.e., fluorocarbon surfactants, to the EDTA-carbonate solvent solution serves to impart a rate of sulfate deposit dissolution that is substantially greater than the rate presently achievable with either EDTA by itself or EDTA in combination with a carbonate, with or without other surfactants.
Fluorocarbon surfactants, i.e., surfactants containing fluorocarbon radicals, in general have been found to be effective for use in the invention, with anionic fluorocarbon surfactants providing the greatest improvement in the extent and rate of sulfate deposit dissolution
4, An aqueous solution of an amino polyacetic acid and a carbonate, such as ammonium carbonate, ammonium bicarbonate, sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate, provides a more efficient calcium sulfate scale removal composition. It is thought that the aminopolyacetic acid reacts with the calcium ions in solution and forms a chelated species. This causes more calcium sulfate to dissolve, and also results in the release of hydrogen ion from the calcium polyacetic acid chelate. This released hydrogen ion reacts with the bicarbonate forming CO2 gas, which then aids in the scale break-up. In addition, surface active agents have been employed in such compositions,
The removal of calcium sulfate scale deposits from surfaces by contacting the deposits with an aqueous solution comprising a chelating or sequestering agent in the form of a polyamino carboxylic acid, a soluble carbonate or bicarbonate and a fluorocarbon surfactant. The solution is permitted to contact and react with the deposits at a pH of approximately 6 to 10 and at ambient or higher temperatures, depending upon the sufficiency of the reaction rate. The concentration of the polyamino carboxylic acid can be from a minimum of 1% to maximum solubility, the concentration of the carbonate is based upon the concentration of the polyamino carboxylic acid and maybe on the order of one mole of carbonate to one mole of acid, and the concentration of the fluorocarbon surfactant can be 0.002 weight percent or above.
The soluble carbonate may include any soluble carbonate or bicarbonate, such as (NH4)HCO3, NaHCO3, KHCO3, Na2 CO3, (NH4)2 CO3, K2 CO3 and mixtures thereof.
The chelating agent may be any suitable polyamino carboxylic acid such as EDTA, NTA (nitrilo triacetic acid), DTPA (diethylenetriamine pentaacetic acid), HEDTA (N-hydroxyethylethylenediamine triacetic acid), and mixtures thereof.
The concentration of the fluorocarbon surfactant in the overall composition can be a minimum of approximately 0.002 weight percent. The preferred concentration is between approximately 0.005 and 0.01weight percent. The optimum pH of the composition is between approximately 6 and 10.
The dissolution of the sulfate scale deposits can be achieved at a temperature of ambient or above, depending upon the reaction desired and the nature of the surface being cleaned. Higher temperatures may be utilized to increase the reaction rate,
