Degradation rates and mechanisms of acid-resistant coatings in copper-leaching tanks

Modern society is reliant on high purity metals, making mineral processing and refining of metals vitalto everyday life. However, unit operations, associated with the extraction and purifucation of metals, can come into contact with highly acidic substances, which may lead to corrosion of tanks, reactors and process equipment. To reduce the required maintenance, operational downtime, and thereby the overall production costs, steel and concrete surfaces need to be protected from these chemicals. Organic coating technology has been proved to provide protection against acidic environments, but its potential in the mineral industry has not yet been thoroughly investigated. This particular industry poses unique challenges, with high operational temperatures (around 75 °C) and combined acidicerosive environments.
The use of organic coatings to protect tanks, pipes, and secondary exposure areas, may provide significant savings compared to utilizing ceramic or metal alloys with similar acid resistance. Nevertheless, organic coatings are often disregarded in favor of their inorganic counterparts. This is in part due to a lack of knowledge regarding the limitations of organic coatings, and the inability to accurately predict coating lifetime. More know-how is required before organic coatings can compete against materials with a proven track-record.
In this PhD project, state-of-the-art literature on acid resistant organic coatings was collected and reviewed. The subsequent investigations concerned predicting coating lifetimes and investigating degradationmechanisms in the harsh acidic environments found in agitated leaching reactors, wherefailure can occur due to chemical reactions, erosive wear, acid diffusion, or a mixture of the three. Theresearch conducted can be summarized by the five main parts below.
Part I An in-depth literature study was performed to uncover and review uses and limitations ofacid-resistant coatings in the chemical industry, with a comparison to alternative resistant materialsbased on metals and ceramics. In addition, coating degradation phenomena caused by acid exposure, were mapped to the extent possible, and analysis methods for detecting coating degradation type and severity were listed and discussed. It was found that more knowledge on chemical and physical degradation mechanisms was required. Improvements in resistance to elevated temperatures and abrasion would decrease the risk of use and increase the potential application areas of organic coatings exposedto acidic environments in the chemical industry.
Part II A series of thermoset coatings with some degree of acid resistance were selected for sulfuricacid and water immersion experiments: one vinyl ester, one polyurethane and three different novolacepoxies. The measured outputs were weight change, visual observations, FTIR, and SEM/EDS analysis of coating cross-sections. One of the selected novolac epoxies failed so rapidly in sulfuric acid, that it was excluded from further experimentation. The two other novolac epoxy coatings showed a fast and high weight gain, alongwith delamination or blistering in the warm acid, but no hard evidence of chemical changes. The polyurethane coating showed a gradual drop in weight due to acid exposure, caused by the dissolution of limestone fillers. The vinyl ester coating showed little weight gain, no deformation, and no EDS detectable sulfur element in the film after 53 days of immersion.
Part III To explore the performance of the coatings in a realistic environment, a pilot-scale agitated leaching reactor containing sulfuric acid and micron-size ore particles, was designed and constructed. The most important scaling factor was the stirring intensity. Failure, attributed to erosion, was observed via dry film thickness change during exposure. This outputparameter was found to be a function of film swelling and contraction, due to chemical exposure, as well as the polishing caused by the erosive particles. For coating samples placed on the reactor bottom, film reduction rates varied with radial position. Maximum rates were found about halfway between reactor center and wall. Polishing rates also varied significantly with acid concentration, most likely due to chemical degradation of the coatings which damaged the surface mechanical properties. The vinyl ester-based coating was most resistant to the simultaneous erosive/acidic exposure; a 1000 µm thick film would have an estimated lifetime of 5:9 ± 1:1 years, compared to a similar novolac epoxy orpolyurethane coating where the lifetime was estimated to 1:6 ± 0:2 and 1:4 ± 0:1 years, respectively.
Part IV A series of newly designed and constructed diffusion cells were used to measure sulfuric acid diffusion rates through the coatings. A mathematical model was developed to simulate the experimental data, and diffusion mechanisms were studied through the modeling results.It was found that sulfuric acid deteriorated the coating barrier properties as it diffused through the films. This was expressed in the modeling results, where the diffusion coefficient required continuousconcentration dependency, combined with a simplified three-step time dependence, to accurately simulatethe acid breakthrough time and the subsequent steady state flux. Using the model developed, itwas possible to estimate a coating lifetime, based on the acid breakthrough time. Coating degradation reactions with the sulfuric acid proved the most significant factor in determining barrier properties.A vinyl ester-based coating proved the most effective barrier to sulfuric acid diffusion, where a 735µm thick film showed no acid permeation after 118 days of exposure. Comparatively, a 1000 µm thickpolyurethane coating was simulated to last 278 ±13 days, a 100% solids amine-cured novolac epoxywould last 56±3 days, and a regular amine-cured novolac epoxy would last 3.2 ±0.5 days before acid permeated the film.
Part V A summary of coating performance was provided, and the lifetime of the coatings in the agitated leaching environment was evaluated based on the diffusion, reaction, and erosion failure mechanisms.The vinyl ester was the peak performer in terms of chemical inertness, barrier properties and erosive resistance. The polyurethane came second, while novolac epoxies had the poorest performance of the coatings investigated. The combined polishing and acid diffusion would yield lifetimes below the previous estimate from diffusion cells experiments, due a reduction of the effective coating thickness, but diffusion barrier properties were found to be the most significant factor. For coatings in agitated leaching reactors, the importance of considering all degradation mechanisms was emphasized, sincethey showed synergistic behavior: ionic diffusion enabled chemical reactions in the film, the following chemical reactions speeded up acid diffusion rates, the combined reaction/diffusion increased theerosion rates, and finally erosive wear would speed up diffusion rates.