Back end operations

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

An obvious method of increasing the filtration area in the vessel is to stack several plates on top of each other the plates are operated in parallel. One design, known as the plate filter, uses circular plates and a stack that can be removed as one assembly. This allows the stack to be replaced after the filtration period with a clean stack, and the filter can be put back into operation quickly. The filter consists of dimpled plates supporting perforated plates on which filter cloth or paper is placed. The space between the dimpled plates and the cloth is coimected to the filtrate outlet, which is either into the hoUow shaft or into the vessel, the other being used for the feed. When the feed is into the vessel, a scavenger plate may have to be fitted because the vessel will be full of unfiltered slurry at the end of the filtration period. This type of filter is available with filtration areas up to 25 m and cakes up to 50 mm thick. 

Development of molybdenum electrodes in the 1950s permitted the use of electrically assisted melting in regenerative furnaces (81). In the 1990s, approximately one-half of all regenerative tanks ate electrically boosted. Operating practice has shown that effective use of electricity near the back end of the furnace, where the batch is added, can reduce fossil fuel needs. This lowers surface temperature and reduces batch volatilisation. 

Under normal operating conditions, the concentration of the trioxide is unlikely to exceed 10 ppmv, but this is sufficient to elevate the acid dew point to around 422 K (300°F). This places a limit on the lowest acceptable back-end temperature if acid condensation and resulting corrosion problems are to be avoided. 

Increased thermal efficiency is possible if boiler back-end temperature is reduced. Theoretically, without modifying boiler output, a reduction of 8 °C (14.4 °F) in back-end temperature results in a saving in fuel oil consumption of 0.3%. Low temperature corrosion can occur, however, when boilers are operated with back-end temperatures close to the S03 dew point. 

Separation processes, as could be seen from Figure 2.1, position themselves at the back end of the sequence in operations in the mineral processing flowsheet. The front-end operations has been found virtually to terminate with the liberation or the size-reduction processes involving crushing and grinding. It is important to limit the amount of size reduction to that at which adequate liberation is accomplished. The term adequacy is related to the cost involved in comminution and to performance of the concentration methods that follows. The concentration is obtained by separation processes which rely on differences in the properties of the particles, the physical and physico-chemical characteristics of minerals. In this context, it will only be relevant to refer to Table 2.5 which presents a summary of the processes along with the properties of the minerals that are exploited. 

The back end of the piston is fitted with an adjustable stop to limit the total travel of the piston. The hydraulic pump is operated a few feet away from the tracer injector so that oper-... 

The diagram in Figure 16.1 shows two possible paths for this cycle, that is, with and without fuel reprocessing. The majority of reactors in the world and all U.S. reactors operate with a once-through cycle without reprocessing. Some countries, particularly France, do fuel reprocessing with reuse of the plutonium from spent fuel. The portions of the cycle, that precede the introduction of the fissile material into the reactor are referred to as the front end of the cycle, while the back end includes those steps that occur after the removal of the fuel from the reactor. The details of this cycle and the chemistry involved are discussed below. 

In the last few years, a third type of microfiltration operating system called semi-dead-end filtration has emerged. In these systems, the membrane unit is operated as a dead-end filter until the pressure required to maintain a useful flow across the filter reaches its maximum level. At this point, the filter is operated in cross-flow mode, while concurrently backflushing with air or permeate solution. After a short period of backflushing in cross-flow mode to remove material deposited on the membrane, the system is switched back to dead-end operation. This procedure is particularly applicable in microfiltration units used as final bacterial and virus filters for municipal water treatment plants. The feed water has a very low loading of material to be removed, so in-line operation can be used for a prolonged time before backflushing and cross-flow to remove the deposited solids is needed. 

Arizona Portland. Arizona Portland operates a cement plant in Rillito, Arizona, near Tucson, which has been burning tdf since 1986. They have a different kiln configuration from Genstar and they use a different tdf feed system. They feed the tdf into the flash calciner at the back end of their system. Until recently, they had been using an air lock system to feed tdf into their calciner at a rate of 2 tons per hour. However, they had problems with the pneumatic feed system. Pieces of steel wire from the tdf repeatedly plugged up the elbows in the feed path. Because of these problems they decided to install a mechanical feed system instead. 

A distinguishing feature of naphtha operations relative to gas feedstock crackers is the production of heavier products. This requires a considerable amount of process plant at the front end and the back end of the plant to handle them. A typical process block flow is illustrated in Figure 9.1. 

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