Calcium carbonate sludge

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. 

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]. 

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 %. 

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. 

Modifications of the basic process are undersoftening, spHt recarbonation, and spHt treatment. In undersoftening, the pH is raised to 8.5—8.7 to remove only calcium. No recarbonation is required. SpHt recarbonation involves the use of two units in series. In the first or primary unit, the required lime and soda ash are added and the water is allowed to settie and is recarbonated just to pH 10.3, which is the minimum pH at which the carbonic species are present principally as the carbonate ion. The primary effluent then enters the second or secondary unit, where it contacts recycled sludge from the secondary unit resulting in the precipitation of almost pure calcium carbonate. The effluent setties, is recarbonated to the pH of saturation, and is filtered. The advantages over conventional treatment ate reductions in lime, soda ash, and COg requirements very low alkalinities and reduced maintenance costs because of the stabiUty of the effluent. The main disadvantages are the necessity for very careful pH control and the requirement for twice the normal plant capacity. 

Tyj)e of dryer Applicable with dry-product recirculation True and colloidal solutions emulsions. Examples inorganic salt solutions, extracts, milk, blood, waste liquors, rubber latex, etc. Pumpable suspensions. Examples pigment slurries, soap and detergents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. does not dust. Recirculation of product may prevent sticking Examples filter-press cakes, sedimentation sludges, centrifuged sobds, starch, etc. 

Of course, this argument is perfectly true where it can be positively demonstrated that MU water requirements really are very low. Once again however, if this is not the case, then—most treatment programs are designed primarily for corrosion control and do not compensate for undue hardness entering the boiler—calcium carbonate scale can and does develop over time. This process takes place even where the MU water is relatively soft, and results in the formation of insulating boiler tube deposits or boiler vessel sludge. 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

PCA 16 is available as Beldene 161/164 (50/35% w/w solids), Acumer 4161 (50%), and Polysperse (50%). These are low-phosphorus content materials that have found application in boiler FW formulations because of excellent sludge conditioning and particulate dispersion properties. The number 16 represents a 16 1 w/w ratio of acrylic acid and sodium hypophosphite, giving PCA 16 a MW range of 3,300 to 3,900. PCA 16 is particularly effective for the control of calcium carbonate and sulfate deposition. It is usually incorporated with other polymers in formulations and is approved for use under U.S. CFR 21, 173.310. 

Determining calcium levels normally does not identify hardness breakthrough because the calcium salt simply reacts with phosphate precipitant (or similar treatment) and is lost as a sludge. It does, however, produce an immediate and noticeable reduction in alkalinity. (Calcium bicarbonate breaks down to calcium carbonate and carbonic acid.)... 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

Calsinter A process for extracting aluminum from fly ash and from flue-gas desulfurization sludge. The ash is sintered with calcium carbonate and calcium sulfate at 1,000 to 1,200°C and then leached with sulfuric acid. Developed at Oak Ridge National Laboratory, United States in 1976, but not known to have been piloted. 

The wastes generated include scmbber water which can contain some particulate and calcium carbonate or other chemical sludges, and various t5q)es of salts, and some dusts from the baghouse and cyclone which are recombined with the soil and are tested for various contaminants. 

Coal liquefaction Fischer-Tropsch synthesis Synthesis of methanol Hydrogenation of oils Alkylation of methanol and benzene Polymerization of olefins Hydrogenation of coal oils, heavy oil fractions, and unsaturated fatty acids Adsorption of S02 in an aqueous slurry of magnesium oxide and calcium carbonate S02 or removal from tail gas Wet oxidation of waste sludge Catalytic desulfurization of petroleum fractions Wastewater treatment... 

Also, bulk water-precipitated calcium carbonate is often found as a sludge, intermixed with rust, silt, biomass, other mineral precipitates, and foulants. 

Calcium phosphate is equally as troublesome as calcium carbonate by forming scales that reduce heat-transfer efficiencies and sludges that foul the system. In addition, calcium phosphate scale is intractable and once formed, deposits can be extremely difficult to remove from dirty cooling systems, so prevention is vital. 

Also, calculate the holding time index. If the holding time is more than, say, 48 hours, expect potential problems with the dispersion and removal of suspended solids and sludges. Also expect potential problems with regard to the holding up of supersaturated minerals in solution, such as calcium carbonate. Again, some innovative solution may need to be found. 

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