Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1997-08-26
1999-05-11
Hruskoci, Peter A.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210738, 210740, 210 961, 210143, 210206, C02F11/14
Patent
active
059024879
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus and processes for controlling the conditioning and dewatering of a suspension which is continuously flowing towards a continuous dewatering process such as a belt press, a belt thickener or a centrifuge.
Such processes conventionally involve flowing the suspension continuously along a flow line past a conditioner dosing point at which conditioner is dosed into the suspension and on to a dewatering stage at which the conditioned suspension is dewatered.
The effectiveness of the conditioner and dewatering is influenced by the dose of the chosen conditioner and so it is necessary that the amount of conditioner that is added should be controlled carefully.
Conventionally several samples of the suspension are taken from time to time and are subjected to laboratory conditioning tests with varying amounts of one or more conditioners. The optimum conditioner and amount for use at the time of the test is selected and the process is then run utilising that amount of that conditioner, until a subsequent set of tests shows that a variation should be made.
Since the suspension is liable to change from time to time, and to avoid the need for repeated manual testing, it is well known to try to control the amount of conditioner automatically. Thus there have been numerous proposals in the literature to observe a parameter of the process and to control the addition of conditioner in response to this parameter. For instance it is known to monitor the in-flowing suspension and to use some parameter of this to control the addition of conditioner. Unfortunately, many of the methods that have been proposed for monitoring a parameter of the in-flowing suspension have again involved off-line analysis, and this is inconvenient. Other methods have involved in-line monitoring of, for instance flow rate or density, but known methods have not proved satisfactory in practice.
It is also known to monitor the performance of the dewatering process by analysing the solids or liquid recovered from the process. Monitoring the solids (for instance the dry matter of a cake) can be rather difficult and so many of the proposals have involved monitoring the turbidity or other properties of the filtrate (eg a centrate).
Although it might be thought that satisfactory control of the addition of conditioner could be achieved by monitoring the performance of the dewatering process (such as filtrate clarity), in fact there is a fundamental deficiency in such processes. Although they operate satisfactory under substantially steady state conditions, they react only slowly to changes in the suspension. This has the result that a considerable volume of suspension may be dewatered before any change indicated by the quality of the filtrate has resulted in a change in the dosing of flocculant and in a modification of the dewatering conditions. Thus, if the suspension is being dewatered under optimum conditions on a substantially steady basis and the suspension then changes, considerable volumes of suspension may have been dewatered before the filtrate quality has resulted in an effective change in the amount of conditioner which is dosed into the suspension. This is a particular problem with the modern tendency for dewatering apparatus such as centrifuges and belt presses to have a very large capacity.
Even when the process is said to be an automated process, all such processes at present generally require at least one operative substantially permanently observing the process so as to be able to apply manual override if there is, as sometimes occurs, a relatively sudden change in the nature of the suspension or if the dosage control proves inadequate.
Increasing the degree of automation in the addition of the conditioner can (subject to the problems discussed above) be advantageous but in practice it still tends to be unsafe to have the process conducted wholly automatically. As an example, the rate of flow into the belt press or centrifuge or other dewatering apparatus will normally have been selected having regard to the
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Polykon Brochure, "The Polykon-Process", A concept for more flexibility and productivity.
Pickering Carl Anthony
Tovey Peter James
Allied Colloids Limited
Crichton David R.
Hruskoci Peter A.
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