Scraper

Scraping, unlike filing or chiselling, is not done to remove a great deal of material. The material is removed selectively in small amounts, usually to give a flat or a good bearing surface. 

A surface produced by machining or filing may not be good enough as a bearing where two surfaces are sliding or rotating. 

The three-square or triangular scraper. Fig. 2.13 is commonly used to remove the sharp edges from curved surfaces and holes, ft is not suited to scraping internal curved surfaces, due to the steeper angle of the cutting edges tending to dig into the surface. However, the sharp point is useful where a curved surface is required up to a sharp corner. 

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When used to separate solid-solid mixtures, the material is ground to a particle size small enough to liberate particles of the chemical species to be recovered. The mixture of solid particles is then dispersed in the flotation medium, which is usually water. Gas bubbles become attached to the solid particles, thereby allowing them to float to the surface of the liquid. The solid partices are collected from the surface by an overflow weir or mechanical scraper. The separation of the solid particles depends on the different species having different surface properties such that one species is preferentially attached to the bubbles. A number of chemicals are added to the flotation medium to meet the various requirements of the flotation process ... 

Pipelines are cleaned and inspected using pigs . Pigs usually have a steel body fitted with rubber cups and brushes or scrapers to remove wax and rust deposits on the pipe wall, as the pig is pumped along the pipe. Sometimes spherical pigs are used for product separation or controlling liquid hold up. In field lines handling untreated crude may have to be insulated to prevent wax formation. 

When only small amounts of sand, wax or scale are experienced the situation can often be contained using wireline bailers and scrapers, run as part of a well maintenance programme. 

A static leaf filter is used for cleaning machine tool coolants. These are used on the suction side of a pump circulating system, with the same pump employed for withdrawal of the filtrate as for backdushing the filter elements. SoHds in this case are removed from the sump by a scraper conveyor. 

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

In conventional disk filters, cake discharge is usually performed by a scraper blade, for cakes thicker than 10 mm, or sometimes by a tapered roU air blowback is often used to assist the discharge. High pressure sprays also have been used for cake discharge. 

The Gaudfrin disk filter, designed for the sugar industry and available in Prance since 1959, is also similar in design to a vacuum disk filter but it is enclosed in a pressure vessel with a removable Hd. The disks are 2.6 m in diameter, composed of 16 sectors. The cake discharge is by air blowback, assisted by scrapers if necessary, into a chute where it may be either reslurried and pumped out of the vessel or, for pasty materials, pumped away with a monopump without reslurrying. 

Cake discharge occurs at atmospheric pressure by the action of a toU or a scraper, assisted by blowback. The cloth may be washed by a spray before the cycle starts again. Filtering areas range up to 8 m and dmm diameters up to 2 meters. The necessity for large seals limits the operating pressure to less than 300 kPa, typically. Cake thickness can be from 2 to 150 mm, depending on machine size, and the speed of dmm rotation up to 2 rpm, usually from 0.3 to 1 rpm. Apphcations occur in the manufacture of pharmaceuticals, dyestuffs, edible oils, and various chemicals and minerals. 

The American version of the dynamic filter, known as the Artisan continuous filter (Fig. 30), uses such nonfiltering rotors in the form of turbine-type elements. The cylindrical vessel is divided into a series of disk-type compartments, each housing one rotor, and the stationary surfaces are covered with filter cloth. The feed is pumped in at one end of the vessel, forced to pass through the compartments in series, and discharged as a thick paste at the other end. At low rotor speeds the cake thickness is controlled by the clearance between the scraper and the filter medium on the stationary plate, while at higher speeds part of the cake is swept away and only a thin layer remains and acts as the actual medium. 

Historically, internal scrapers or paddles were used in some designs, and loose rails were used in others to break up any caking material which formed. The nature of these designs mechanically limited the furnaces to relatively small sizes producing about 3000 t/yr. High maintenance costs were also involved. 

The reaction gas is rapidly quenched with injected water at the point of optimum yield of acetylene, which happens to correspond with the point of maximum soot production. Coke will deposit on the walls of the burner and must be removed from time to time by a scraper. 

The flame-space walls are stainless steel and are water cooled. No mechanical coke scraper is required. A water quench cools the cracked gas stream rapidly at the poiat of maximum acetyleae and this is followed by a secondary water quench. The primary quench poiat can be adjusted for variation ia throughput, to accommodate the depeadeace of acetyleae yield oa resideace time ia the flame space. 

Quarries that excavate soft stone, notably mad or chalk, do not dtiU or blast, but extract the stone usiag heavy-duty rippers and scrapers. In the Middle West and Florida, lake mads and soft coralline limestone are dredged ia a process much like stripmining. 

Flake Mica. Flake mica is mined from weathered and hard rock pegmatites, granodiorite, and schist and gneiss by conventional open-pit methods. In soft, residual material, dozers, shovels, scrapers, and front-end loaders are used to mine the ore. Often kaolin, quartz, and feldspar are recovered along with the mica (see also Clays Silicon compounds). 

The most important stainless steel [12597-68-1] series are the 200-, 300-, and 400-series. The 300-series, primarily 302, 304, and 316, is used in the dairy industry, whereas the 400-series is used for special appHcations, such as pump impellers, plungers, cutting blades, scrapers, and bearings (Table 11). Surface finishes are specified from No. 1 to No. 8 (highly poHshed) the No. 4 finish is most commonly used. 

For close-clearance impellers the correlations in Figure 35b apply to installations without scrapers. If scrapers are used, the values given by the equations should be multiphed by a factor of 1.3. 

The type of varnish used in the process depends on the kraft paper manufacturer and basis weight of the papers the machine, temperature, and control (scraper bars, squeeze roUs) used the method of cutting the paper to size the laminate being produced (post-forming or regular) and the press-cure cycle (see Laminated materials, plastic). 

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