Caking, prevention

Improved mechanical strength (bulking) of the freeze-dried cake prevent powdering. 

Iwasaki, Tetsuji, and Hitoshi Hosokawa. 1989. Caking - Preventing Agent for Fertilizer, U.S. Patent 4,812,158, March 14. 

The Latest in Fertilizer Cake Prevention. 1990. Nitrogen, No. 187, September-October,... 

In situations where a low concentration of suspended solids needs to be separated from a liquid, then cross-flow filtration can be used. The most common design uses a porous tube. The suspension is passed through the tube at high velocity and is concentrated as the liquid flows through the porous medium. The turbulent flow prevents the formation of a filter cake, and the solids are removed as a more concentrated slurry. 

To prevent the solid complex from caking (which occurs if the stirring is not sufficiently rapid) about 75 ml. of dry ethylene dichloride may be added. 

Fats can be an important source of lubrication in the preparation and consumption of foods (30). Marble slabs on which hot candy is poured are lubricated with fat to prevent sticking. Also, bread and cake pans are treated with heatstable edible oil. 

To keep the frequency of backwash and the washwater demand down, and to prevent undesirable cake formation on the filter surface, deep bed filtration is appHed to very dilute suspensions of solids concentrations less than 0.1% by volume. 

Electrophoresis and electro osmosis can be used to enhance conventional cake filtration. Electrodes of suitable polarity are placed on either side of the filter medium so that the incoming particles move toward the upstream electrode, away from the medium. As most particles carry negative charge, the electrode upstream of the medium is usuaHy positive. The electric field can cause the suspended particles to form a more open cake or, in the extreme, to prevent cake formation altogether by keeping aH particles away from the medium. 

The vertical cylindrical vessel of the pressure version of the Moore filter developed in France for the sugar industry, which houses a set of radially arranged leaves, is twice the height of the leaves. This allows the leaves to be raised, rotated, and lowered into the different compartments in the bottom half of the vessel. Positive air pressure must be maintained throughout the operation to prevent cake faH-off, and the cake is blown off the leaves by air blowback. 

Another possibiUty is to enclose only the working, top part of the horizontal belt in a pressure vessel and pass the belt through the sides of the vessel. The operation must be intermittent because the belt cannot be dragged over the support surface with the pressure on, and the entrance and exit ports for the belt must be sealed during operation to prevent excessive losses of air. The movement of the belt is intermittent and is synchronized with decompression in the vessel therefore, the entire vessel volume must be depressurized in every cycle and this is wasteful. There is also an inevitable downtime. There are no problems with discharging the cake because this is done at atmospheric pressure. 

As more and more of the filtrate is removed, the slurry graduaUy thickens and may become thixotropic. The soHds content of the thickened slurry may be higher than that obtained with conventional pressure filtration, by as much as 10 or 20%. A range of velocity gradients from 70 to 500 L/s has been suggested as necessary to prevent cake formation and to keep the thickening slurry ia a fluid state (27). 

The three disadvantages described can be avoided by using soHd elements, instead of permeable ones, which create the shear to prevent or reduce cake formation. Only the stationary surface inside the filter is then available for filtration and this means a reduction in capacity. This is not a problem because the soHd disks can be slimmer and the collection of filtrate does not have to be through a hoUow shaft. 

Shipping and Storage. MaHc acid is shipped in 50-lb, 100-lb, and 25-kg, multiwall paper bags or 100-lb (45.5 kg) fiber dmms. A technical-grade, 50% solution may be shipped in tank cars or tank tmcks. MaHc acid can be stored in dry form without difficulty, although conditions of high humidity and elevated temperatures should be avoided to prevent caking. 

Dicalcium hexakiscyanoferrate [13821 -08 ] Ca2[Fe(CN)g], is formed as yellow crystals by reaction of Hquid or gaseous HCN with iron(II) chloride in water containing Ca(OH)2 or CaCO and having pH > 8. It is used to prevent caking of other substance and serves as a useful starting material in the preparation of other [Fe(CN)g] salts. Examples of mixed salts include calcium dicesium hexakiscyanoferrate [15415-35-7] CaCs2[Fe(CN)g], and calcium dipotassium hexakiscyanoferrate [20219-00-5] CaK2[Fe(CN)g]. 

The cake produced by the digestion is extracted with cold water and possibly with some diluted acids from the subsequent processes. During the cake dissolution it is necessary to maintain the temperature close to 65°C, the temperature of iron sulfate maximum solubiUty. To prevent the reoxidation of the Fe " ions during processing, a small amount of Ti " is prepared in the system by the Ti reduction. The titanium extract, a solution of titanium oxo-sulfate, iron sulfate, and sulfuric acid, is filtered off. Coagulation agents are usually added to the extract to faciUtate the separation of insoluble sludge. 

Calcination of the washed cake of the hydrated gel of Ti02 is carried out in rotary kilns similar to those used for producing cement (qv). The kilns are directiy heated with gas or oil. An excess of air in the kilns is required to prevent the possible reduction of Ti02 to its lower oxides. The presence of such oxides, even in small quantities, gives the final product a bluish gray shade. 

Humectants such as propylene glycol, glycerol, sorbitol, or calcium chloride can prevent drying and caking. 

Poly(vinyl alcohol) is an innocuous material having unlimited storage stabiUty. It is most commonly suppHed in 20-kg, 22.7-kg (50-lb), and 25-kg bags equipped with a moisture barrier to prevent caking. PVA is also available in bulk or in super sacks. The FDA regulations governing the use of PVA are shown in Table 8. Poly(vinyl alcohol) maintains an exemption for tolerance from the EPA. 

The latter method typically requires less severe conditions than the former because of the labile nature of the organic anhydride (87,137). Both of these reactions can result in explosions and significant precautions should be taken prior to any attempted synthesis of a peracid (87). For soHd peracids the reaction mixture can be neutralized with sodium hydroxide and the resulting fUtercake washed with water. In the case of the sulfuric acid mediated reaction the peracid has sodium sulfate incorporated in the cake (135). The water of hydration present in the sodium sulfate is desirable to prevent detonation or deflagration of the soHd peracid when isolated in a dry state (87,138,139). 

---