In the phosphate ore processing industry, scaling is common place due to the high solids loading of process streams. Cleaning is inevitable and causes lost revenue from production downtime. For those cleaning the scaled system, frustration happens when cleaning equipment is unable to reach into the nooks and crannies of the piping and other process equipment. When cleaned inefficiently, these hard-to-reach places often result in reduced production efficiencies. Headaches ensue when reduced operation performance and extra production downtime is not what was expected and planned on paper.
For a well-run production process, three different areas are considered in the initial stage of its conception. These areas could be called engineering, operational, and maintenance. The engineering aspect is the physical design of the system. The operational aspect is the human interaction with the design during the production process, and the maintenance aspect is the planned and unplanned tasks required to keep production at its optimum.
Once the system has been built and becomes operational, maintenance costs can be greater than expected. For systems that develop scale in the process vessels and piping, it can be difficult to remove. This is one reason why it is important in the conception phase of the process to bring all parties together to consider all the possible items that can negatively impact production. Efforts should be taken to prevent or minimise the formation of scale. Once it has formed, there are two options to remove it: mechanically or chemically. Which is of these is better is dependent on various factors. Some of these factors are cost, downtime requirements, potential harm to the equipment, and the effectiveness of cleaning.
In 2017, a customer approached Athlon to enquire whether it was possible to use ion exchange to reduce or altogether remove their scaling issue, which was occurring frequently. For years, the customer would routinely use an outside contractor to hydroblast the equipment, taking 12 hours to clean the vessel and piping. The disadvantages, according to the customer, were the inability of hydroblasting to reach into the corners of the process vessel, to sufficiently clean critical areas of the process vessel, and reach far enough into the process piping to remove its scale. As a result, the process continued to be restricted in recirculation flowrates, higher process pressure drops, and at times still required an unplanned shutdown to clean again. All of these resulted in lower production and increased production costs.
The customer correctly reasoned that by removing or reducing the cation components within the scale, scaling would stop or, at a minimum, the formation rate would drastically decrease. Laboratory analysis indicated the scale was calcium-based in combination with phosphate, sulfate, and fluoride.
Based on the makeup water chemistry, it seemed apparent that ion exchange would not be possible. To confirm this, collaboration was made with two major manufacturers of ion exchange resin. Each manufacturer indicated that the high concentrations of sodium, calcium, magnesium, silica, and sulfates would place a heavy burden on the ion exchange system or would require a system so significant in size to make the operational economics unrealistic.
Analysis of the various mechanical options resulted in the same outcome: the economics of each were unrealistic. With mechanical options ruled out, it became necessary to evaluate a chemical solution. Due to the high total dissolved solids and high particle concentration of the process water, the use of chemicals made it impossible to prevent scaling. Therefore, a cleaning solution that went beyond the current capabilities in hydroblasting needed to be designed.
The chemical cleaner had to clean into the nooks and crannies, meet low corrosion rates, be effective at ambient temperatures, perform the task in the same or shorter period of time as hydroblasting, and be cost-justifiable. Different compounds were therefore evaluated to determine whether they met the requirements of the customer.
Hydrochloric acid, the least expensive pure acid available, is effective for removing phosphates but not recommended for the 304L stainless steel of the system, even when the acid is inhibited. Sulfuric acid poses the problem of adding sulfates, which only reinforces the formation of calcium sulfate scale. These acids are also notorious fumers. Acidic fumes are a great safety concern for everyone present during cleaning. Additionally, corrosion will take place on vapour space surfaces where the acid has recondensed but the corrosion inhibitor would be absent to protect these surfaces. Corrosion in the vapour space leads to reduced equipment integrity over time.
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