Recycling direct

As direct recycling is understood, the as-supplied material is transformed to powder of the same composition by either chemical or physical treatment, or a combination of both. Prerequisites for direct recycling are  

Typical examples, which will be discussed in more detail below, are the zinc process, or the Coldstream process for reclamation of cemented carbide scrap, and the oxidation and subsequent reduction of tungsten heavy metal turnings or thoriated tungsten (see Table 11.3). 

Direct recycling is combined with a minimum of energy consumption, chemical waste, and lowest production costs. A general comparison of direct and indirect tungsten recycling is presented in Fig. 11.3. 

We are still far away from full utilization of the direct recycling potential, because huge amounts of the corresponding materials are still chemically converted to APT. The reasons for that situation are  

Zinc Bloating/crushing Hardmetal powder Sorted hardmetal pieces 

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Direct recycle Dispersion Electrostatic precipitation Filtration Needle bonded fab Reverse jet Reverse pressure Shaker type ric... 

Absorption Destruction Direct recycling Dispersion Total enclosure Gas Wet scrubbing... 

Destruction Direct recycling Electrostatic precipitation Filtration Gravity settlement Total enclosure Wet scrubbing... 

Recycle refers to the utilization of a pollutant-laden stream (a source) in a process unit (a sink). Each sink has a number of constraints on the characteristics (e g, flowrate and composition) of feed that it can process. If a source satisfies these constraints it may be directly recycled to or reused in the sink. However, if the source violates these constraints segregation, mixing, and/or interception may be used to prepare the stream for recycle. 

On the source-sink mapping diagram, sources are represented by shaded circles and sinks are represented by hollow circles. Typically, process constraints limit the range of pollutant composition and load that each sink can accept. ITie intersection of these two bands provides a zone of acceptable conqKisition and load for recycle. If a source (e.g., source a) lies within this zone, it can be directly recycled to tiie sink (e.g., sink S). Moreover, sources b and c can be mixed using the lever-arm principle to create a mixed stream that can be recycled to sink S. 

In order to reduce fresh-water consumption in the scrubber, the usage of distillation bottoms and the off-gas condensate should be maximized since diey have the least ammonia content. The flowrate resulting from combining these two sources (5.8 kg/s) is sufflcient to run the scrubber. However, its ammonia composition as determined by the lever-arm principle is 12 ppm, which lies outside the zone of permissible recycle to the scrubber. As shown by Fig. 4.7, the maximum flowrate of the off-gas condensate to be recycled to the scrubber is determined to be 4.1 kg/s and the flowrate of fresh water is 0.9 kg/s (5.8 — 0.8 — 4.1). Therefore, direct recycle can reduce the fresh-water consumption (and consequently the... 

Developing strategies for segregation, mixing, and direct recycle... 

Figure 7.12 Structural representation of segregation, mixing and direct recycle options.

Figure 7.13 is structural representation of segregation, mixing, and direct recycle candidate strategies for the problem. Each source is split into several frac-tions that can be fed to a sink. The flowrate of the streams passed from source w to sink u is referred to as The terms F, Z", and represent the inlet flowrate, inlet composition, and outlet flowrate of the streams associated with unit u. Since mixing is embedded, there is no need to include the mixing tank (m = 4) or the source that it generates u> = 5) in the analysis. Unless recycle of biotreatment effluent is considered, there is no need to represent the biotreatment sink in Fig. 7.13. However, streams allocated to biotreatment should be represented and their flowrates are referred to as (m = 5 is the biotreatment sink). Finally, fresh water may be used in any unit at a flowrate of Fresh,.

These results can be used to construct the solution as shown in Fig. 7.14. The target for minimum CE discharge through segregation, mixing and direct recycle is 0.488 X 10 kg/s (about 15 kg/yr). The solution indicates that the optimal policy is to segregate the effluents of the two scrubbers, pass the effluent of the first scrubber to the reactor, recycle the aqueous effluent of the reactor to the hrst scrubber and dispose of the second scrubber effluent as the terminal wastewater stream. 

Figure 7.13 Solution to the CE case study using segregation, mixing and inter-process direct recycle.

It is interesting to compare the optimal configuration shown in Fig. 7.16 to end-of-pipe solutions. Suppose we retain the identified segregation, mixing, and direct recycle strategies shown in Fig. 7.14, but intercept the wrong stream. For instance, if the CE content of the terminal wastewater stream is to be reduced from 6.5 ppmw... 

Pyroredox is currently under development. Several problems have been identified and work is underway to overcome these obstacles. As mentioned previously, this process has immediate application as a primary plutonium purification process and as a means to process spent electrorefining anodes for direct recycle back to electrorefining. 

Onsite processing by agglomerating or briquetting and directly recycling back through the EAF (to concentrate the zinc content)... 

Mass balance constraints (6.1), (6.3) and (6.5) need to be reformulated to account for the water from storage. The water into a unit in this case is not only comprised of freshwater and directly recycled/reused water, but also water from storage. This... 

The mass balances that are first considered are those that deal with the mass flow around a unit. The first of these is an inlet water balance, given in constraint (7.1). The water into a unit is the sum of the directly recycled/reused water, freshwater and water from the various storage vessels. The outlet water balance is presented in constraint (7.2). This constraint states that water leaving a processing unit is either directly recycled/reused, discarded as effluent or sent to one or more storage vessels... 

The constraints that comprise the scheduling module of the model are divided into four groups, namely, task scheduling, direct recycle/reuse scheduling, storage scheduling and time horizon constraints. 

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