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Organic VS Aqueous Mixing (5 replies)

Zander Barcalow
11 months ago
Zander Barcalow 11 months ago

What’s the difference between organic continuous and aqueous continuous mixing in solvent extraction? And which of them is better?

David Kano
11 months ago
David Kano 11 months ago

Organic continuous occur when droplets of aqueous are dispersed in an organic phase. This continuity is generally stable at a mixer ratio greater than 1. And usually reduce organic in aqueous phase (electrolyte and raffinate), this continuity is better for copper SX and EW operations. Meanwhile aqueous continuous occurs when droplets of organic are dispersed in an aqueous matrix. It generally stable at mixer ratio of less than 1.

Marshal Meru
11 months ago
Marshal Meru 11 months ago

When mixing two immiscible phases, one contains the other. If the organic contains dispersed aqueous droplets called organic continuity (O/C) in the otherwise aqueous called continuity (A/C). In copper SX he oxides in solvent the organic continuity has a high contents of droplets of aqueous and low content of organic entrainment. The aqueous continuity has high content of droplets of organic and low content of aqueous entrainments.

Which is better? The answer is related to the mixer settler stage objectives. For example in the case of copper SX, the "terminals" stages operate in organic continuity because these stages produce raffinate and advance electrolyte and " the objectives of these stages" are reduce the organics losses or organic entrainments in raffinate and electrolyte.

The aqueous continuity operations is better for decrease the content of impurities of leaching in electrolyte (aqueous entrainments) or decrease the electrolyte losses in stripping stages.

Paul Morrow
11 months ago
Paul Morrow 11 months ago

The previous two comments provide a good answer but perhaps I can add a little. As mentioned, in an emulsion of two immiscible phases, one phase will be present as droplets dispersed in the other phase. In a glass of beer, the continuous phase is the beer; the dispersed phase is the gas bubbles, so we can say this is beer continuous.

The dispersed phase is present as droplets which can be very fine and so difficult to coalesce and separate from the continuous phase. So, in the phases that come out a settler, the continuous phase can have microscopic droplets of the dispersed phase entrained in it. The dispersed phase usually separated much more cleanly while there is a greater chance that the continuous phase will contain entrained droplets of the dispersed phase. So, in a copper solvent extraction it is, if possible desirable to have aqueous continuous in the stage where loaded organic is produced to minimize aqueous entrainment in the advancing organic phase. Similarly, it is desirable to have organic continuous in the stage where raffinate is produced to minimize organic losses out with the raffinate.

Which is better in terms of phase separation rates depends on the system. With some systems like copper, the differences are very slight to negligible certainly in the laboratory. Organic continuity will cope better with dirty aqueous feeds (high suspended solids), but of course, that is not addressing the problem which is that you should have a clear PLS with low TSS. In other systems, phase continuity can be critical. With a somewhat turbid aqueous feed in a uranium amine circuit, aqueous phase continuity results in an emulsion that will not separate - your settler fills with pudding. But organic phase continuity in the mixer permits satisfactory operation.

How do you set the phase continuity? First, the major phase is the one that usually wants to be continuous. So, if you O/A ratio coming into the mixer is 2/1, the O will probably be continuous and vice versa. But, this is not a hard and fast rule. Phase continuity tendencies also depend on relative phase densities and interfacial tension. So, you can, in some systems, operate aqueous continuous even with O/A = 2. If you want the minor phase, the one with the lower advance flow to be the continuous one, you usually accomplish that by recycling some of that phase from the settler back to the mixer. You also often do this in mixer-settlers where your advance phase ratio is high, say over 3/1 (O/A or A/O) so that you can have more efficient phase mixing conditions which are best around O/A = 1 in the mixer.

To set the continuous phase in a laboratory mixer, you typically start the mixer in the phase you want continuous and then physically lower the mixer into the middle of the mixer box, or as low as necessary to have the required pumping. In an SX plant, you unfortunately cannot do that, so what is usually done is that the dispersed phase flow is stopped and only the continuous phase is allowed to advance and fill the mixer boxes until the desired phase continuity is present. Then, the minor phase flow is turned back on. You can also try to pump the minor phase out of the mixer box before you start up but whether you can do that depends on the plant design. It can be a mess.

Some years ago, Cytec produced two short video clips that illustrate the difference between aqueous continuous and organic continuous separation in a copper system. In particular, you can see how different the interface appears, bubbly with organic continuous, flat with aqueous continuous and you can also see that with organic continuous, the separated aqueous is a lot clearer showing that there is, as we expect, less entrained organic.

Carl Jenkins
11 months ago
Carl Jenkins 11 months ago

We found that in some processes the mass transfer rate depends on the continuity for the same process conditions - O: A advance (not the O: A mix), temperature etc. There are many factors that may cause this, and it is not easy to discriminate between them. Among the factors - partition coefficient, different diffusion in continuous and dispersed phases, physic-chemical properties of the interface etc.

As an example - we encountered a system where the average drop size is much smaller with organic continuous for the same liquids, conditions and mixing system. Thus, the mass transfer efficiency is 10-20% better for organic continuous.

Tony Verdeschi
11 months ago
Tony Verdeschi 11 months ago

Excellent description. I have had very little to do with SX so this was an interesting read. I particularly liked the beer analogy - giving a real world 'visual' example that a lay person can identify with is an excellent way of getting technical issues across.

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