Application of modified water glass acid-resistant concrete in the copper metallurgy industry

Thread Content

Modified water glass concrete is a new type of corrosion-resistant concrete that is developed on the basis of water glass concrete by adding modifiers to increase its density and improve its resistance to acid penetration; it is also known as dense water glass concrete. 　　Apart from having poor corrosion resistance to hot phosphoric acid, hydrofluoric acid, and higher fatty acids, modified water glass concrete exhibits good corrosion resistance to most inorganic acids, organic acids, and acidic gases. It also possesses good heat resistance, with a usable temperature of up to 900°C. Our company has been using it in projects such as floor construction, pickling tanks, and beam and column reinforcement since 1987; some of these applications are still in good condition after 12 years. 1 Primary raw materials 1.1 Sodium silicate The main technical parameters are modulus and density; the appropriate density ranges from 1.35 to 1.45 g/cm3, and it can also be expressed in Baumé degrees. The relationship between the two is density = 145 / (145 – Baumé degree) (g/cm3). A more suitable modulus is 2.6 to 3.0, and the modulus and density can be adjusted as desired. 1.2 Sodium fluosilicate Sodium fluosilicate is a curing agent; its technical specifications require a purity of over 95% and a moisture content of less than 1%. 1.3 Powder material The powder material serves as a filler for acid-resistant concrete, with technical specifications requiring an acid resistance level of over 94% ； Moisture content less than 0.5% ； Acid-resistant impurities must not be present ； Fineness of 1600 pores/cm2 with a sieve residue of not more than 5%, or 4900 pores/cm2 with a sieve residue of 10% to 30%. 　　Common powder materials include cast stone powder and quartz powder, or a mixture of the two; for applications requiring high temperature resistance, refractory clay brick powder is used. 1.4 Aggregates Aggregates constitute the main framework of acid-resistant concrete, directly affecting its physical and chemical properties as well as its technical specifications. Apart from the fineness, everything else is the same as that of powder materials. The commonly used aggregate is quartz stone (sand); for applications requiring high temperature resistance, fragments of refractory clay bricks are used. Coarse and fine aggregates should have a good gradation. 1.5 Modifiers The main function of modifiers is to improve density and impermeability; they are important components in modified water glass concrete, and the technical requirements are as follows. 　　(1) It should be soluble in water or able to disperse uniformly in water glass solution under vigorous stirring. 　　(2) It will not cause water glass to coagulate or separate. 　　(3) It should have a certain degree of corrosion resistance or polymerize into a solid when exposed to acid. Common modifiers are furan-based organic monomers and furan resins, such as furfural, furfuryl alcohol, or mixtures of furfuryl alcohol and furfural, with a dosage of 1% to 5% (by weight) of the amount of water glass used. 2 Construction Process 2.1 Mixing First, add the dry materials to the mixer and mix them dry for 2 minutes; then add water glass and modifiers, and mix them wetly for 3 minutes before discharging the mixture. To prevent stalling, the mixing volume should not exceed 40% of the mixer’s capacity. The discharge must be carried out quickly and promptly; if the mixer malfunctions, the contents inside it should be removed immediately and cleaned with water. 　　After work each day, the modified concrete inside the machine and on the mixing blades must be cleaned thoroughly. When the volume of work is small, manual mixing can be used. 2.2 Pouring 　 Apply a layer of thin mortar on the construction surface; wait for it to cure before pouring, and try to avoid leaving construction joints. When a construction joint must be left, it should be formed as an inclined joint with a rough surface. The next time joining the joints, apply thin mortar first and then pour the material; the joint area should be compacted and smoothed out. 　　During the hot periods of summer and autumn, modified water glass concrete begins to set and take shape within a few minutes; therefore, it is necessary to compact and level it quickly on site. It is advisable to achieve a smooth surface before the concrete starts to set, so that it can be formed in one go, thereby avoiding wrinkles that may appear after setting and could affect its appearance as well as its resistance to water penetration and corrosion. 2.3 Form removal and curing During the curing period, contact with water or steam, as well as very dilute acids, is strictly prohibited. Generally, it is carried out according to Table 1 based on the ambient temperature, form removal time, and curing time. Table 1: Demolding and Curing Times for Modified Water Glass Concrete
Environmental temperature (°C): Demolding time (days): Curing time (days)
15 ≮ 42: 20–25 ≮ 21; 25 and above ≮ 17.24

Quality inspection: The quality criteria for modified water glass concrete are an open porosity of ≤5%, a water resistance strength of >15 MPa, and a compressive strength of ≥20 MPa. 　　Check for honeycombing, pitting, cracks, deformation, etc.; if any are present, they should be repaired. 2.5 Acidification treatment Acidification can be carried out using 20%–40% sulfuric acid, 15%–20% hydrochloric acid, or 15%–30% nitric acid, with the treatment being repeated 3–4 times. Apply the treatment evenly every 8 hours or more, sweeping away the white crystalline powder that has precipitated in each application. 2.6 Precautions 　　(1) The optimal construction temperature is 18–30°C. If the temperature is too high, the reaction proceeds too rapidly, making construction extremely difficult. Ice water can be used to lower the temperature of the water glass, or construction can be carried out at night, or the amount of curing agent used can be reduced to slow down the reaction rate. 　　When the temperature is below 10°C, the setting time increases, making it prone to flow, deformation, and cracking; preheating the aggregates or using heated curing methods can be employed. 　　(2) Water glass can cause burns to the eyes and skin. Sodium fluosilicate is toxic; therefore, workers working near the mixer should wear protective goggles, rubber gloves, and masks. 3 Engineering Applications 3.1 Floors and Gullies Our company first applied this material on a large scale to the floors of the electrolysis workshops in copper smelting plants in 1987; the area covered was approximately 3,300 m2 with a thickness of 80 mm. This solution eliminated the problem of acidic sludge and water flowing around on the floors of those workshops, as well as the issue of tiles falling off. The floors were exposed to water and an electrolyte containing 200 g/L of sulfuric acid, yet to this day, the vast majority of those floors remain in good condition. 　　Since 1991, it has been successively adopted in the sulfuric acid plant of our company’s copper smelting plant, the electrolytic copper plant of the Northwest Copper Processing Plant, the Third Smelting Plant, and the lead-zinc smelting plant, significantly reducing the costs associated with ground anti-corrosion measures; in most cases, it has performed well. Some of them became damaged within less than two years of use, due to the construction party’s failure to follow the proper construction procedures – such as not using any modifiers, employing sand, gravel, and aggregates with extremely low acid resistance, and arbitrarily adjusting the amount of water glass used. 3.2 Acid and heat-resistant integrated underground flues The flue gases in the underground flues of smelting plant chimneys reach temperatures of 200–250°C, and contain substances such as SO2, dust, and water vapor. The original flues were constructed with ordinary concrete, lined with refractory bricks and acid-resistant tiles; they required partial demolition and repair every few years. This approach posed significant construction challenges, resulted in poor corrosion protection, and was costly. Later, it was modified by binding a steel mesh to ordinary concrete and pouring it as a single unit using modified water glass concrete, thereby achieving integration of high-temperature resistance and corrosion protection; the cost was only 1/3 of the original. It has been in use for nearly 10 years now without any signs of damage. 3.3 Reinforcement of beams and columns Due to corrosion, some beams and columns in the electrolysis and sulfuric acid production areas of our company’s copper smelting plant have become weak and deteriorated; in some cases, the rebar inside them is exposed, posing a serious safety hazard. When reinforced with ordinary concrete, in addition to the need for anti-corrosion treatment, the construction time is also long, making it unsuitable for emergency repairs and maintenance. Later, the method of adding welded rebar and using modified water glass concrete for reinforcement was adopted, thereby integrating anti-corrosion and load-bearing functions in a single step and reducing the construction time. It has been in use for 7 years now and remains in perfect condition. 3.4 Integral Electrolyzer The integral electrolyzer is cast using modified water glass concrete, and successful results have been achieved after several years of trial use. Compared with electrolyzers with concrete lining anti-corrosion coatings or fully fiberglass-reinforced plastic structures, it has significant advantages. 　　(1) Integration of corrosion resistance, load-bearing capacity, and liquid containment; simple to manufacture, fast and convenient, with one-time molding. 　　(2) The cost is only 1/3 of that of the full-fabricated fiberglass type, and it is also lower than the cost of the lined type. 　　(3) No maintenance is required over a long period of use, with a lifespan of over 8 years. 　　Key points for pouring modified water glass concrete electrolyzers: First, high-quality materials should be used, with good aggregate gradation; the amount of cast stone powder used should also be increased appropriately ； Secondly, use vibrators with excellent performance to ensure that all areas are compacted properly ； Third, to slow down the solidification rate, it is advisable to carry out the process during the cooler months of spring or indoors ； Fourth, the surface finish must be good, without any honeycombing or pitting. 　　If potassium water glass is used for water glass, its impermeability and corrosion resistance are better. 3.5 Special sealing for leak repair When a container used to store concentrated sulfuric acid leaks, the usual approach is to empty the sulfuric acid and then seal the leak by welding in steel plates. The downside of this method is that it requires a large amount of storage space to hold the sulfuric acid, consumes energy, takes a long time to carry out repairs, and disrupts production. Using modified water glass putty for application allows for sealing leaks without shutting down the system or draining it, and it can cure within a few minutes in high temperatures. To prevent the mortar from being washed away by rainwater or dilute acids, a two-layer fiberglass protective layer can be applied once the leak has been completely sealed and no more leakage occurs. This saves time and effort, allows for immediate sealing of leaks, and does not disrupt production. 3.6 Anti-corrosion and insulation composite layer for external heat exchangers in the sulfuric acid production system The steel shells of the external heat exchangers in our company’s sulfuric acid system have not been fitted with any anti-corrosion measures for decades; as a result, the condensed acid generated by the smelting flue gas, along with that flue gas itself, causes severe corrosion to the steel walls. In particular, at the areas where the inlet openings expand, it is necessary to remove large sections of the insulation layer during the two inspections held each year, as well as cut away the corroded and perforated steel sections before welding in new steel sections and applying new insulation layers – an approach that results in significant waste. 　　In the past two years, water glass mortar has been used to carry out anti-corrosion treatment on welds and certain parts of the steel wall that are prone to corrosion, from the inner side. The outer wall is first bonded with water glass slurry and glass fiber cloth to create 4–5 layers of anti-corrosion coating, which are then secured with steel wires. An anti-corrosion resin coating is applied as the top layer, and an insulation layer is placed on top of the anti-corrosion coating. After 2 years of operation, no signs of corrosion or leakage of smoke have occurred.