Tailing stream

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

Fig. 3. The beneficiation of taconite ore on Ihe iron range requires large volumes of water in concenlrating by magnetic separation. To eliminate massive waste-disposal problems, huge thickeners, such as the 300-foot (91,5-meter) diameter caisson unit shown here, are used. This system will handle over 70 million gallons (265 million liters) per day or 50,000 gallons (189,250 liters) per minute of liquid and 250 tons per day of suspended solids. Clarifying the waste tailing stream permits reclamation of water on a large scale for plant reuse...

Example. A tailings stream from the hot-water flotation process (oil sands) contains 27 % (mass) solids. Estimate the suspension viscosity. Light-scattering analysis indicates that the particles are finely divided and anisotropic with a 10 Tm major dimension and a 0.5 pm minor dimension. Using Eq. 

Although the details are beyond the scope of this book, health problems can be caused by solids and liquids suspended in water (for example, in waste-tailings streams) or in air (for example, in stack-emission plumes). Specific potential hazards have been associated with a diverse spectrum of colloidal materials, including synthetic chemicals, coals, minerals, metals, pharmaceuticals, plastics, and wood pulp. Limits for human exposure for many particulate, hazardous materials are published [504,505],... 

In many surface-separation processes, there will occur three distinct phases or process streams a product stream (either oil or water), a waste (tailings) stream, and an interface or rag layer emulsion stream, which may contain emulsified oil and/ or water. The interface emulsion can be the most troublesome, in terms of process operation, and the most complex and intractable, in terms of treatment. Mikula shows (Figure 1 in Ref. [66]) a photomicrograph of a quite stable interface emulsion (rag-layer emulsion) in which one can clearly observe the simultaneous occurrences of both O/W and W/O emulsions in different regions of the same sample. 

The third step in the hot-water process is scavenging. Froth flotation with air is usually employed. The scavenger froth is combined with the separation-cell froth to be further treated and upgraded to synthetic crude oil. Tailings from the scavenger cell join the separation-cell tailings stream and go to waste. 

One of the greatest problems that has emerged from tar sand resource development is the disposal (and control) of the tailings streams that arise from the hot-water separation. The extent of this problem had not been appreciated until the commercialization of the tar sands commenced. 

The clay discharged as part of the tailings stream does not settle and, therefore, limits the... 

The unit cost of enriched uranium in the form of UFj depends on the U content of the uranium, the price paid for the natural uranium from which the uranium was enriched, the cost of the separative work expended in enriching the uranium, and the composition of the tails stream containing depleted uranium leaving the uranium enrichment plant. The procedure for calculating the cost of enriched uranium is described in Chap. 12. The unit costs... 

Some relations for isotope separation plants are simpler when expressed as weight, mole, or atom ratios, defined as the ratio of the fraction of one component to the fraction of a second. These ratios are denoted by Greek letters f,, or r) for feed, depleted, or enriched stream, corresponding to z, x, or y. In a two-component mixture, these ratios are defined as the ratio of the fraction of the desired component to that of the other component. For example, in a tails stream, the weight, mole of atom ratio for a two-component mixture is... 

The simplest type of separating unit or stage is one that receives one feed stream and produces one heads stream enriched in the desired component and one tails stream depleted in the desired component. Figure 12.9 shows such a stage, which is fed at rate Z with z fraction desired component and which produces a heads stream at rate M with y fraction desired component and a taik stream at rate N with x fraction desired component. Flow rate and composition should be on the same basis, e.g., weight, mole, or atom. 

A cascade like Fig. 12.12, in which no attempt is made to reprocess the partially depleted tails streams leaving each stage, will be called a simple cascade. In a simple cascade the feed stream for one stage is the heads stream from the next lower stage of the cascade. This type of cascade connection is used in the lower stages of the Norsk Hydro electrolytic heavy-water plant where the tails streams have too little deuterium to warrant processing for deuterium recovery. The theory of such a cascade is developed in Sec. 6. 

The streams that move away from the ends of the cascade, that is, the tails stream in the enriching section and the heads stream in the stripping section, are known as reflux. 

In the simple cascade of Fig. 12.12, whose performance was illustrated in Fig. 12.14, it is impossible to obtain high recovery of desired component because of losses in the tails streams leaving every stage. Desired component in these streams can be recovered by recycling these... 

The heads stream and tails stream fed to each stage have the same composition ... 

In the ideal cascade discussed up to this point, each stage receives as feed two streams of the same composition, a tails stream from the stage next higher in the cascade and a heads stream from the stage next lower in the cascade. In such a cascade the heads separation factor /, tails separation factor 7, and overall separation factor a are related by... 

To permit operation at a cut different from j while still ensuring that the composition of heads and tails streams entering each stage be equal requires a more complex cascade... 

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