Carbonation sludge

Pre-liming. Lime slurry, 0.25% lime on juice (0.250 g of CaO/100 g juice), is added to bring the pH of the mixture into the alkaline range. Insoluble calcium salts are precipitated as finely dispersed coUoids. Calcium carbonate in the form of recycled first carbonation sludge is added to provide coUoid absorption and stabilization. Temperature may be cool (50°C) or hot (80°C) depending on the temperature of the next step, or occasionally on the type of diffusion equipment. Retention time is 15 to 30 min. 

Internal treatment was often based on recommendations dating from the 1920s. The deliberate addition of soda ash (sodium carbonate) to the BW to form carbonate sludges (rather than, say, sulfate scales) that could be removed by BD was a common program. If unde-... 

The most popular form of internal treatment for many years dates from the nineteenth century and is based on a combination of soda ash and caustic. This traditional program, the so-called carbonate-cycle or carbonate treatment, utilizes the addition of sodium carbonate to the BW to deliberately form carbonate sludges that can be removed by BD, rather than permit sulfate scales to develop. If sulfate scales do form in the boiler, the removal process is difficult and very time-consuming, and obviously, boiler operating efficiency will continue to decrease as the sulfate scale increases in thickness. 

In addition to the formation of calcium carbonate sludge, some of the carbonate decomposes giving sodium hydroxide, which precipitates magnesium bicarbonate, and chloride, etc. as the hydroxide and is then incorporated into the carbonate sludge. Silicates are also often removed in the process as serpentine. Additional caustic may also need to be added, depending on the natural alkalinity present in the MU water. 

Calcium hardness precipitates as calcium carbonate sludge... 

Probably the rotary horizontal kiln is the most versatile, since it allows a feed of lumps or fines of limestone or marble, or wet or dry calcium carbonate sludges (Fig. 7.1). The main component of this calcination system is a 2.5- to 3.5-m diameter by 45- to 130-m long firebrick-lined inclined steel tube. Heat is applied to the lower end of this via oil, gas, or coal burners [7]. The feed to be calcined is fed in at the top end. Slow rotation of the tube on its axis gradually moves the feed down the tube, as it tumbles countercurrent to the hot combustion gases. In this way, wet feed is dried in the first few meters of travel. Further down the tube, carbon dioxide loss begins as the temperature of the feed rises. By the time the solid charge reaches the lower, fired end of the kiln it reaches temperatures of 900-1,000°C and carbon dioxide evolution is virtually complete. Normally the temperature of the lower end of the kiln is not allowed to go much above this as it reduces the life of the kiln lining. It also adversely affects the crystal structure of the lime product since it produces a dead-burned or overburned lime. Overburned lime is difficult to slake to convert it to calcium hydroxide and raises... 

Emissions to water in addition to the figures given EniChem is authorised to dump 3300 tonnes/annum of calcium carbonate sludge into the sea. 

Salt crystals, rock salt, sand, wool Brick clay, kaolin, crushed sand Pigments, paper, soil, worsted wool fabric Several foods, copper carbonate, sludges Chrome leather, vegetables, fruits, gelatin, gels... 

One of the driving forces for developing new designs of kiln is that substantial quantities of calcium carbonate are available in a finely divided form. The sugar and paper/wood pulp industries, for example, produce a calcium carbonate sludge (see sections 30.4 and 32.16), which can be calcined and recycled. Many plants use rotary kilns for this purpose, while some use fluidised bed kilns [16.6,16.48,16.49]. 

Many sugarbeet processing plants operate lime kilns on-site to produce both the quicklime and the carbon dioxide required for the purification process. Shaft kilns are generally used, burning lump limestone. A few plants calcine the dried carbonation sludge, using rotary and circular multiple hearth kilns [30.6]. 

Pulp mills use considerable quantities of hypochlorite for bleaching. Both sodium and calcium hypochlorites are used, produced by reacting chlorine with caustic soda and milk of lime respectively. It is reported, however, [32.29] that the trend is towards the use of sodium hypochlorite which produces soluble wastes, whereas calcium hypochlorite produces a calcium carbonate sludge. 

In order to cut costs at the lime sooftening plant, the operator decides to recalcinate the calcium carbonate sludge according to the reaction... 

Gypsum and Calcium Carbonate Sludge, Because of their improved quality, gypsum and calcium carbonate sludge are no longer dumped, but are supplied to the cement industry for use as raw materials. Also, the quantity of gypsum produced has been reduced by 38 %. 

Carbon dioxide may be supplied as the liquid and then vaporized in standard apparatus for injection into the treatment tanks, or it may be recovered from a flue gas. In the latter case, the presence of impurities is an obvious concern. In a few plants, carbonate sludge is digested with acid to regenerate the CO2 value, and this gas can also be used. 

Precipitation treatments should be used for boiler pressures up to 61 to 68 bars (900 to 1000 psig). The two common techniques use phosphate control or carbonate control. These treatments involve the formation principally of calcium phosphates or carbonate sludges, their dispersion by various organic chemicals, and, finally, their removal by blowdown. 

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