Analytical techniques for identifying mineral scales and deposits

Abstract

In many industrial processes, the feed water used contains mixtures of dissolved ions that are unstable with respect to precipitation. Various factors such as pH, temperature, the type and concentration of dissolved ions, flow velocity, equipment metallurgy, and so on contribute to the precipitation and deposition of sparingly soluble salts on equipment surfaces. The class of crystalline and amorphous compounds formed in industrial water systems, generically known as scale and deposits, has a widespread importance across a variety of disciplines, as can be seen from other chapters in this book and from other books [1-3]. Scale is defined as the deposit of certain sparingly soluble salts such as calcium carbonate, calcium phosphate, calcium oxalate, magnesium hydroxide, and calcium sulfate from the process fluids after precipitation onto the tubing and other process surfaces. The commonly encountered deposits in industrial water systems include carbonates, sulfates, and phosphates of alkaline earth metals, silica, magnesium silicate, corrosion products, microbiological mass, and suspended matter. These deposits, especially on heat-transfer surfaces in thermal distillation, cooling, and boiler systems, lead to overheating, loss of system efficiency, unscheduled shutdown, and untimely heat exchanger failure. In desalination by reverse osmosis (RO) process, the deposition of unwanted precipitates may result in poor water quality and premature membrane failures. The deposition of scale in some cases may be beneficial as in the case of drinking water transmission lines wherein the layer of scale deposit protects the piping from corrosion by isolating it from the water. However, in most cases, scale is undesirable as it adversely affects the overall efficiency of the process.