RU2386729C2 - Scale removal method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention related to removal of scale from carbon steel and can be used for flushing processes of heat engineering equipment - steam boilers and communication systems. Method for removing scale from carbon steels involves etching of scale with the solution containing 0.002-0.005 mol/l of EDTA or OEDF complexon, 0.01 - 0.1 mol/l of protonated complex Fe(II) with EDTA or OEDF, 1-2 g/l of sulphuric acid and water, and having pH 2.2 when using EDTA or pH 1.5 when using OEDF, at 60-110 °C and rapid mixing of solution with mixer rotation frequency of 1000 rpm; at that, etching of scale is performed only the solution is blown down with nitrogen or 0.01 g/l of hydrazine or hydroxylamine is added to it.

EFFECT: method provides complete removal of scale from surface of steel equipment at maintaining physic and chemical properties of metal and at increasing removal speed of oxide-salt deposits from surface; at that, waste regenerated solution can be used for many times.

3 dwg, 1 tbl, 4 ex

Description

The invention relates to the field of hydrometallurgy, in particular to leaching iron from oxide-ore technogenic raw materials, removing scale from carbon steel, and can be used for washing heat power equipment (steam boilers and communication systems).

A known method of removing scale (patent RU 2169794 C1), which consists in the use of a multistage method of cleaning metal surfaces from deposits using HEDP and sodium sulfite in an alkaline environment. The disadvantage of this method of removing scale is the increase in embrittlement of the metal due to hydrogenation of steel associated with the use of sodium sulfite. The use of non-optimal modes of descaling dramatically increases the time of removal of deposits up to 48 hours.

Patent RU 2119553 C1 proposes a complex solution composition containing expensive substances (maleic, sulfomaleic acid, mono (alkyl, dioxyalkyl) sulfomaleic acid esters) that are not produced in Russia in large volumes, which sharply increases the cost of the method used. In this application, it is proposed to use purified substances, which is not economically viable. In addition, the disadvantages of the method include the impossibility of using waste solutions. The conditions proposed in the invention do not correspond to the optimal mode of descaling (φ = 0.2 V and pH 2).

The closest technical solution, selected as a prototype, is a method of descaling using a solution for cleaning steel and brass surfaces of thermal power equipment (patent SU 1805687 A1), which consists in using EDTA and hydrogen peroxide in an acidic environment.

In the known method of removing deposits, selected as a prototype, an increase in the dissolution rate of the deposits is achieved by using EDTA complexones in a pH of 8-8.5 at 40-50 ° C.

The disadvantage of this method is the creation in solution of a high positive potential, which complicates the dissolution of iron oxides. To create a solution in the invention, the use of chemically pure substances is necessary, which sharply increases the cost of the method. In addition, these solutions are recommended to be used only once, with the subsequent removal of the spent solution.

Studies have shown that the most effective way to remove scale from carbon steels is to use regenerated protonated iron (II) complexonates with EDTA (or HEDP) at pH 1.5-3.5 in deaerated solutions.

The invention solves the problem of the complete dissolution of scale from the surface of steel equipment while maintaining the physicochemical parameters of the metal and increasing the rate of leaching of iron oxides from various oxide-ore phases.

The problem is solved by using a protonated complex (Fe (II) with ethylenediaminetetraacetic acid (EDTA) or hydroxyethylidene diphosphonic acid (HEDP)) of the composition FeHY ^- (where Y is the acid anion in the case of using EDTA of the composition HC ₁₀ N ₂ O ₈ ^3- , in the case of using OEDP composition C ₂ O ₇ P ₂ H ₄ ^3- ) in a concentration of from 0.01 to 0.1 mol / l (2-20 g / l) containing EDTA (OEDP) in the composition within a concentration of 0.002 to 0.005 mol / L (5-20 g / L), H ₂ SO ₄ 0.01-0.02 mol / L (1-2 g / L), hydrazine 0.0003 mol / L (0.01 g / l). The proposed method allows to increase the rate of removal of oxide-salt deposits from the surface by 10-100 times (graphs 1, 2). The method allows the use of spent solutions, conducting their regeneration. Regeneration consists in reducing Fe (III) to FeHY ^- and creating a pH of 1-3. Using this method allows you to speed up the process of removing deposits up to 1 hour and increase economic efficiency.

Based on the conducted experimental studies, the optimal technological conditions for the etching of scale from the surface of steel equipment were determined. The optimal pH when using the proposed method is 1.5 (when using OEDP) or 2.2 when using EDTA (set by the addition of sulfuric acid) at T = 60-110 ° C. Etching is carried out in a nitrogen atmosphere created by purging or adding a hydrazine reducing agent (N ₂ H ₄ ) in an amount of 0.0003 mol / L (0.01 g / L).

The conditions of the experiment to substantiate the optimal modes of use of this method: pH 1-3.5; m (Fe ₃ O ₄ ) = 20 mg, Vp-pa 100 ml, particle radius = 0.27 μm, surface 9.7 m ² / g, T = 60 ° C, in a nitrogen atmosphere while mixing the solution with a stirrer speed ω = 1000 rpm

The claimed technical solution differs from the prototype in that in the proposed method of descaling it is proposed to use the FeHY ^- complex, which exhibits strong reducing properties, restoring the surface of Fe ₂ O ₃ , Fe ₃ O ₄ to Fe (OH) ₂ . The proposed method allows multiple use of the spent regenerated solution of complexonates, and not pure, which is more effective. After removing deposits, the washing solution is heated to 130-150 ° C in order to form a protective magnetite film on the steel surface.

The disadvantages of this method include the fact that it is applicable only to closed systems in which the FeHY ^- complex ^is unstable in the oxygen atmosphere, or requires the use of a reducing agent (hydrazine or hydroxylamine).

The optimal conditions for the application of the invention are illustrated by the following graphs:

Chart 1. The pH dependence of the dissolution rate of magnetite in sulfuric acid at an EDTA concentration of 0.01M (1), HEDP-0.1M (2) -0.01M (3), H ₂ SO ₄ control without the addition of complexones (4) .

Chart 2. The effect of the concentration of EDTA (2) and FeHY ^- (1) on the dissolution rate of magnetite at pH 3.5; T = 60 ° C.

Chart 3. Determining the region of existence of the FeHY ^- complex on the E - pH diagram (Purbe diagram).

From the analysis of the data it follows that the optimal pH of the use of the solution for EDTA is pH 2.2, and for HEDP, pH 1.5. With increasing concentration of complexones, the rate of dissolution increases proportionally.

From the experimental results presented in graph 2, it can be seen that the dissolution rate of magnetite from the concentration of EDTA passes through a maximum attributable to C _EDTA = 0.003 mol / L. The additions of the protonated complex increase the dissolution rate of the Fe ₃ O ₄ sample, while the limiting value of the velocity is observed, which is described by an equation of the form:

,

,

where W _on is the experimental value of the specific dissolution rate;

W ₀ is the reaction rate constant;

Fe (II) HY ^- , H ₃ Y ^- , А ^-  are the equilibrium concentrations of ions, respectively, Fe (II) HY ^- , H ₃ Y ^- and the anions A ^- (in this case, SO ₄ ^2- );

Ka ₁ , Ka ₂ , Ka ₃ are the reciprocal of the equilibrium constants upon adsorption of A ^- , Fe (II) HY ^- , H ₃ Y ^-, respectively.

To justify the optimal conditions for the dissolution of magnetite, a thermodynamic approach is proposed for the optimal mode of existence of the protonated complex using the E-pH diagram presented in Figure 3.

From an analysis of the data in graph 3, it follows that the optimal conditions for descaling by the proposed method are in the range of pH 1-3 and the redox potential from 0.2 to 0 V. This potential value (E) can be created by purging the system with nitrogen or adding reducing agents : hydrazine (or hydroxylamine) 0.0003 mol / L.

An example of a specific implementation of the claimed method

Thus, the use of the proposed economical and high-quality method of descaling allows to increase the rate of removal of oxide-salt deposits from the surface by 10-100 times in comparison with the use of pure complexone. At the same time, the spent regenerated solution of complexonates can be used repeatedly, which is cost-effective, provides a minimum level of metal corrosion during etching and ensures that all the physicochemical properties of the metal are unchanged, increases the durability of the equipment and its performance.

Claims (1)

A method for removing scale from carbon steels, including etching of a scale with a solution containing 0.002-0.005 mol / L complexon EDTA or HEDP, 0.01-0.1 mol / L protonated complex Fe (II) with EDTA or HEDP, 1-2 g / l of sulfuric acid and water and having a pH of 2.2 when using EDTA or pH 1.5 when using HEDP, at 60-110 ° C and vigorously stirring the solution with a stirrer speed of 1000 rpm, while the etching of the scale is carried out after purging the solution nitrogen or adding to it 0.01 g / l of hydrazine or hydroxylamine.

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