RU2185334C2 - Way of electrochemical treatment of hydrothermal heat transfer agent - Google Patents

Abstract

FIELD: technology of complex utilization of geothermal resources, purification of sewage, geothermal power engineering. SUBSTANCE: liquid solution of hydrothermal heat transfer agent is treated by direct current by method of electric coagulation with use of anodes made of soluble metal with anode density of current 50-200 A/sq m before backward pumping into reservoir. Technical result lies in precipitation of amorphous silica, prevention of supersaturation ( reduction of concentration ) of silica in order to decrease risk of formation of solid deposits in heat power machinery and in holes of backward pumping, generation of additional heat and electric energy from liquid phase of heat transfer agent. Recovered silica can be used as additional mineral product. EFFECT: decreased risk of formation of solid deposits in heat power machinery, generation of additional heat and electric energy.

Description

 The invention relates to methods for the deposition of silica from a hydrothermal coolant, as well as to methods for extracting useful chemical compounds from a hydrothermal separatum for the purpose of complex use together with the energy component.

The essence of the invention lies in the fact that the separated liquid phase of the hydrothermal coolant (an aqueous solution of spent coolant) before being pumped back into the rocks of a natural reservoir is proposed to be treated with direct electric current in an electrolytic cell with soluble metal anodes. The technical result achieved by the implementation of this method is the coagulation and precipitation of amorphous silica, the elimination of supersaturation of the solution for this component in order to reduce the risk of formation of solid deposits in heating equipment and re-injection wells, to obtain additional amounts of electrical and thermal energy and the accumulation of additional mineral product in the form amorphous silica. The extracted amorphous silica, if it satisfies the relevant physicochemical parameters, is a mineral raw material. In this way, you can process a hydrothermal solution having a total silica content of SiO ₂ from 600 to 1000 mg / kg or more.

The scheme for using high-temperature (over 250 ^o C) liquid-phase geothermal resources at different fields looks the same. In production wells, the coolant, which was initially in a liquid state at 250-300 ^o С, comes in the form of a steam-water mixture from the reservoir to the surface, then the steam-water mixture is separated into steam and liquid (separate) in the separators of the geothermal power station. Steam is supplied to the turbine to receive electricity, and the separator can be used to produce heat and electricity. After use, the separator must be returned to the tank through re-injection wells (reinjection wells) to maintain pressure in the tank, production rate of the wells and the duration of the field’s operation and to reduce the environmental impact.

The solution of the hydrothermal separator has a multicomponent composition and when pressure and temperature decrease during production and operation, solid deposits are released from it, which are deposited on the inner surface of the heating equipment and in the wells. The composition of solid deposits includes calcium carbonate, iron oxides and sulfides, and silicon dioxide SiO ₂ (silica). In most cases, amorphous silica makes up the bulk of deposits and creates significant technical difficulties that appear in almost all hydrothermal deposits, which reduces the efficiency of exploitation of geothermal resources.

Initially, the silica in the aqueous solution is in monomeric form (in the form of individual silicic acid molecules H ₄ SiO ₄ ), then when moving in the producing well due to supersaturation through amorphous silica, the polymerization reaction and the formation of colloidal particles develop. After polymerization, the concentration of monomeric silica SiO ₂ depends on the temperature of the solution and at 100 ^o C is about 400 mg / kg

Solid silica deposits in wells and heat exchangers complicate the use of the separate, reduce the amount of heat and electricity received due to the need to re-inject the separatum at a high temperature of 140 ^o C and above. The growth of deposits in reinjection wells leads to downtime in the operation of the Geo-TPP.

The processing of the separate by the claimed method can be carried out at a temperature below the traditional temperature of the reinjection (140 ^o C). The deposition of silica from the hydrothermal coolant stream by electrocoagulation will reduce the return temperature of the separate to 100-80 ^o C and below, due to this to obtain additional electricity and heat in the binary installation, as well as to accumulate an additional mineral product. Amorphous silica can find application in the production of rubber, plastics, cement, glass, paper, sorbents, etc. As a result, it becomes possible to increase the profitability of the use of geothermal energy.

 Several methods are known for treating a hydrothermal separatum with the aim of extracting amorphous silica, which make it possible to reduce the rate of growth of solid deposits and control this process: the method of fluidized substrates using fine-grained sand [1], ultrafiltration using ultramembranes and obtaining monodisperse silica sol, diluting the separatate with steam condensate, methods using reagents. Separate processing technology has been developed at geothermal deposits in Iceland, USA, Japan, Mexico and New Zealand.

Methods using reagents are most developed. As reagents are used: 1. Acid Hcl (also geothermal gases СО ₂ , H ₂ S) for acidification and inhibition of the reaction of polymerization of silica in solution and to reduce the growth rate of deposits; alkali NaOH to alkalize and increase the solubility of amorphous silica; 2. Precipitating agents, leading to coagulation or flocculation and precipitation of silica. A known method of using as an inhibitor of the growth process of solid deposits of silica organic additives with low molecular weight [2].

 In contrast to the analogue method [1] (Axtmann, R.C. Desilication ofgeothermal water. U.S. Patent 4378295, 1983), the method of silica deposition by electrocoagulation excludes the use of complex complex instrumentation and enables automated and controlled reduction of silica concentration in the hydrothermal solution to a predetermined value.

 The disadvantage of the methods for processing the separat using reagents is the need for transport, accumulation and storage of a large number of corrosive chemicals. The required specific consumption of Hcl acid can be up to 300 mg / kg, precipitants up to 700-1000 mg / kg, which corresponds to the annual consumption at the geothermal power plant of several hundred or thousand tons. In addition, the consumption of reagents significantly depends on the temperature of the processed separat and its chemical composition, which is different in each field.

To study the dependence of the residual concentration on the duration of treatment and the measurement of energy consumption, experiments were performed on electrocoagulation in samples of a well separatum from the Mutnovsky field. The average silica content of SiO ₂ in the supersaturated separator of the Mutnovsky wells is 780.2 mg / kg. The typical chemical composition of the samples of the separates treated by electrocoagulation is (mg / kg): NH ₄ ⁺ - 0.7, Na ⁺ - 239.9, K ⁺ - 57.0, Ca ⁺² - 4.0, Mg ⁺² - <0.24, Cl ^- - 291.1, SO ₄ ²⁺ - 124.9, HCO ₃ ^- - 43.9, CO ₃ ^2- - 18.6, F ^- - 1.6, H ₃ VO ₃ - 65.3, Li ⁺ - 1.6, SiO ₂ - 800, the pH of the separatethe 8.6-9.5 (at 20 ^o C). With increasing temperature of the separate to 100-160 ^o With the pH decreased to 7.5-8.0.

The hot solution was processed near the well at a temperature of 60-70 ^o C and the cooled solution in the laboratory at a temperature of 20 ^o C. The electrical circuit of the experiments included a direct current source, a voltmeter and an electrolyzer. The voltage value ranged from 4-30 V, the current strength was 0.5-1.5 A, the current density was 50-250 A / m ² , the distance between the electrodes was 5-15 mm, the temperature of the solution was from 20 to 70 ^o FROM.

The cell had a cylindrical body with a convex bottom, transparent walls made of polyethylene and a plastic stand. Solid metal electrodes of flat rectangular plates 1.5 mm thick were fastened with two pairs of insulating movable inserts, with which the distance between the plates was changed. The area of the part of the electrode plates immersed in the solution varied within S = (0.006-0.0133) m ² = (0.6-1.33) dm ² . The sample volume of the solution processed in the electrolyzer was 1 liter.

 In experiments on the deposition of silica, electrodes made of galvanized steel, stainless steel, copper and aluminum were used. Precipitation on aluminum electrodes was most effective.

The effective size was estimated by the sedimentation rate of the flakes. Silica flakes deposited from the solution using aluminum electrodes had an effective size of 34 to 80 μm and a settling rate of 13-20 mm / min. The settling time of flakes with such dimensions in a sump 0.2 m high at a temperature of 20 ^o C is about 10-15 minutes.

 We have revealed a feature of the process of electrochemical coagulation and deposition in a hydrothermal solution with a high content of silica (600-1000 mg / kg or more). The process has three stages. At the first initial stage, the total silica content decreases slowly. The amount of coagulant entering the solution at this stage is small and its coagulation ability is not large. In the second stage, the deposition rate of silica increases significantly. The second stage is the most important for the technology of deposition of silica. In the third stage, the concentration becomes less than the equilibrium solubility of amorphous silica and slowly decreases with increasing processing time.

With a current strength of 1.0 A, an area of aluminum electrodes S = 1.33 dm ² and a current density of 75 A / m ^{2, the} dependence of the residual total concentration of silica SiO ₂ (the sum of the concentrations of colloidal and monomeric silica) on the processing time t _ET was as follows: t _ET minutes - 800 mg / kg, t _ET minutes - 681 mg / kg, t _ET = 10 minutes - 461 mg / kg, t _ET = 15 minutes - 247 mg / kg, t _ET = 20 minutes - 94 mg / kg, t _ET = 25 minutes - 61 mg / kg, t _ET = 30 minutes - 41 mg / kg.

Data on the concentration of silica in soluble form showed that during electrocoagulation, not only colloidal particles of polymerized silica are precipitated, but also monomeric silica. The ratio of the total concentration and the concentration of monomeric silica remains almost constant and equal to 6-8 during processing until the total silica content is greater than 118-120 mg / kg, which corresponds to the solubility of amorphous silica at a temperature of 20 ^o C. Then this ratio increases sharply up to 40.

The average value of the voltage at the electrodes U during processing at a current strength of I = 1.5 A, S = 1.33 dm ² , current density j = 112 A / m ² and a distance between aluminum electrodes of 10 mm was 10.7 V, the power the cost of electricity for processing N _EL = 16 watts. To reduce the concentration of silica SiO ₂ from 800 to 250 mg / kg (corresponds to the solubility of amorphous silica at an experimental temperature of 60-70 ^o C), the required processing time t _ET was 12.5 minutes, the specific energy consumption (per 1 kg of solution) Q _EL = 0.00333 kW • h / kg. The average voltage value at I = 1.0 A and j = 75 A / m ² was U = 8.22 V, t _ET = 15 minutes, power-N _EL = 8.22 W, Q _EL = 0.00205 kW- hour / kg. At I = 0.5 A and j = 37.5 A / m ^{2, the} voltage U was equal to 4.6 V, t _ET = 26.6 minutes, N _EL = 2.3 W, Q _EL = 0.00102 kW- hour / kg. Thus, the energy consumption for processing decreases with decreasing current strength, but the process duration t _ET increases.

During experiments with a hot solution, it was found that the specific conductivity of a hydrothermal solution σ as a second-type conductor increases with temperature: at I = 1.5 A and j = 112 A / m ² and a temperature of 20 ^o С - σ = 1.07 Ohm ^-1 • cm ^-1 , at a temperature of 55 ^o C - σ = 1,71 Ohm ^-1 • cm ^-1 . The largest contribution to the conductivity of the solution (total 97.9%) yield ions Na ⁺ - 33,24%, K ⁺ - 6.82% Cl ^- - 39.85%, HCO ₃ ^{- -} 2,04%, SO ₄ ^2- - 13.23%, СО ₃ ^-2 - 2.73,%.

In this case, the process of silica deposition by electrocoagulation in a hydrothermal solution at a temperature of 20 ^o C occurs somewhat faster than at a temperature of 50-60 ^o C (10-15%). However, the ohmic resistance when processing a cold solution is much higher than hot. Therefore, the processing of cold separates requires significantly greater energy costs.

The specific consumption of anode aluminum (per 1 g of precipitated silica SiO ₂ ) with a current strength of 1.0 A, S = 1.33 dm ² and a current density of 75 A / m ² is 124-155 mg / g. According to x-ray phase analysis, the deposited material has an amorphous structure. The ratio of the weight fraction of aluminum Al and iron Fe to the fraction of silicon dioxide SiO ₂ in the precipitate varied in the range of 0.1-0.25 depending on the processing time. The proportion of the remaining components was small (weight percent): TiO ₃ -0.3, MnO-0.03, MgO-0.03, CaO-0.15, Na ₂ O-6.4, K ₂ O-0.9 .

Taking into account the experimental data on the duration of the process, the consumption of electricity and anode metal, the optimal values of the current density for processing the hydrothermal separator by electrocogulation in order to reduce the silica concentration to a predetermined value are in the range from 50 to 250 A / m ² .

Literature
1. Axtmann, RC Desilication of geothermal water. US Patent 4,378,295, 1983.

 2. Dubin, L. Silica inhibition: prevention of silica deposition by addition of low molecular weight organic compounds. US Patent 4,532,047, 1985.

Claims (1)

The method of deposition of silica from the liquid phase of the spent hydrothermal coolant, characterized in that the coolant is treated by electrocoagulation using soluble metal anodes at a current density of 50-250 A / m ² .