Environmental Water Treatment Basics: The Meaning and Role of Sludge Drying
Drying equipment for high-viscosity sludge generally uses hot-air rotary-blade dryers. Depending on sludge viscosity, a twin-shaft rotary-blade dryer (vertical or horizontal) or a triple-shaft rotary-blade dryer (horizontal) may be used, capable of drying high-moisture material with an initial moisture content below 85 percent down to a final moisture content below 12 percent in a single pass, with a short drying time of typically 2 to 10 minutes. Because high-speed agitating blades stir the damp material vigorously, the wet material makes good contact with the hot air. Moisture evaporates rapidly in the front half of the machine and the hot-air temperature inside drops sharply, so even when high-temperature hot air is used the temperature rise of the material remains modest. Typical applications include drying of electroplating sludge, paper mill sludge, oilfield sludge, chemical sludge, mineral slurry, fruit pomace, bean dregs, sugar residue and peat, as well as drying of high-moisture materials such as rare earths and of muddy, powdery, granular and flaky high-moisture materials including organic and inorganic compounds and chemical raw materials.
The most commonly used sludge drying equipment is the single-pass rotary dryer, which suits municipal sludge drying. The single-pass rotary dryer is a new type of equipment developed from the ordinary rotary dryer. Fitted internally with a deagglomeration mechanism, movable grate-type wings, a cleaning device and a crushing device, it can dry viscous materials that an ordinary rotary dryer cannot handle. Given that sludge has a certain viscosity and a small particle size, the single-pass dryer is the best choice. It uses a co-current process and a specially designed feeding device so that the moment sludge enters the dryer it exchanges heat rapidly with high-temperature flue gas and forms a hard shell on its surface, greatly reducing adhesion to the shell and material build-up and blockage. In the first quarter of the dryer, besides lifting flights, a deagglomeration mechanism that intensifies evaporation and linked grate wings are installed; these not only conduct heat but also prevent sludge from sticking to and blocking the shell and the lifting flights, making full use of the high drying rate at the feed end and achieving rapid drying by peeling the material layer by layer.
Ordinary sludge drying equipment uses hot air generated by a coal-fired hot blast stove as the drying heat source. Drying efficiency is low and costs are relatively high, the flue gas after drying carries a great deal of water vapour, and problems arise because sludge particles cannot be separated well, leading to excessive total dust content and excessive odour concentration.
Steam from cogeneration plants is also used as a drying heat source, and drying efficiency improves markedly compared with boiler exhaust gas. Exhaust gas from the drying process is treated separately by biological deodorisation towers, physical adsorption and similar means. Limited by the price of steam, this route likewise faces the problem of relatively high cost.
The nationally patented, genuinely energy-saving boiler flue gas sludge dryer uses boiler exhaust gas as the heat source for the sludge drying equipment. Using boiler exhaust gas in this way delivers high drying efficiency and low drying cost, reduces environmental pollution, and answers the national call for energy conservation and emission reduction.
Besides boiler flue gas and steam, heating methods such as natural gas firing and electric heating are also available.
