Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...
Reexamination Certificate
2001-11-09
2003-09-16
Lovering, Richard D. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous liquid or supercritical phase: colloid systems;...
Aqueous continuous liquid phase and discontinuous phase...
C106S464000, C106S465000, C516S928000
Reexamination Certificate
active
06620856
ABSTRACT:
This application is a 371 of PCT/GB 99/04249, filed Dec. 23, 1999, which claims priority based on United Kingdom 9828514.1 filed Dec. 23, 1998 and United Kingdom 9910038.0 filed Apr. 30, 1999.
The present invention relates to the production of concentrated carbonate suspensions, especially calcium carbonate suspensions.
This invention concerns a cost effective process for increasing the solids concentration of a dilute aqueous suspension of a particulate carbonate especially calcium carbonate such as precipitated calcium carbonate, to form a concentrated suspension which is, on the one hand, sufficiently fluid to enable it to be pumped and delivered through pipes or hoses, but, on the other hand, sufficiently viscous to prevent the formation of a sediment of the coarser particles present.
An important factor to be considered if a suspension of particulate carbonate is to be transported in the form of a concentrated aqueous suspension, is that the freight charges will generally be based upon the total weight of the suspension, and it is therefore desirable to minimise the weight of water, and maximise the weight of particulate carbonate in the suspension. Generally, it is found that the solids concentration of the concentrated suspension should be in the range of from about 65% by weight to about 80% by weight of particulate carbonate on a dry weight basis. The calcium carbonate may be prepared in the form of a dilute suspension For example, calcium carbonate prepared by a synthetic route is generally precipitated under conditions such that the product of the precipitation step is a dilute aqueous suspension having a solids content in the range of from about 5% to about 40% by weight. Most frequently, the solids concentration of this suspension will be in the range of from about 15% to about 25% by weight.
A process which has been proposed for increasing the concentration of a dilute aqueous suspension to form a concentrated suspension suitable for transport and storage comprises dewatering the dilute suspension in a tube pressure filter of the type described in GB-A-1240465. This type of pressure filter can be operated at a pressure of up to about 100 bar or more, and can dewater a dilute suspension of precipitated calcium carbonate to produce a cake containing from about 70% to about 74% by weight of calcium carbonate. The cake thus formed must then be mixed with an aqueous solution of a dispersing agent for the calcium carbonate to form a fluid, Theologically stable suspension. A process of this general type is described in EP-A-0768344. Unfortunately this process suffers from the disadvantages that, firstly, the tube pressure filter has been found to be expensive in both capital and maintenance costs, and, secondly, that a considerable amount of energy is required to mix the cake which is formed by the tube pressure filter with a solution of a dispersing agent in order to form a fluid and rheolgically stable suspension.
Other methods have been proposed, e.g. as in U.S. Pat. No. 4242318, to concentrate dilute carbonate dilute suspensions but these do not attain the highest solids concentration possible.
The present invention concerns a method for economically producing a dewatered aqueous suspension of a particulate carbonate which suspension is fluid and Theologically stable and contains a high solids Content which is at least 65%, and is preferably from 70% by weight to about 80%.
According to the present invention there is provided a method of producing a rheologically stable concentrated aqueous suspension of a particulate alkaline earth metal carbonate which method comprises the following steps:
(a) preparing a dilute aqueous suspension of the carbonate having a solids content of not more than 40% by weight;
(b) dewatering the dilute aqueous suspension to form a carbonate suspension having a solids content in the range of from about 45% to about 65% by weight;
(c) optionally mixing with the dewatered suspension formed in step (b) a dispersing agent for the carbonate to form a fluid suspension;
(d) further dewatering the fluid suspension formed in step (b) by thermal evaporation under reduced pressure to raise the solids content of the suspension by a further differential amount of at least 5% by weight; and
(e) after at least some of the dewatering in step
(c) treating the suspension by a mechanical working process in which at least 1 kW.hr per tonne of carbonate on a dry weight basis is dissipated to re-fluidise the suspension.
Dispersing agent is preferably added before step (d) in the method of the invention, but, if not added at this stage, is preferably added prior to or during working in step (e). If dispersing agent is added prior to evaporative dewatering in step (d), further dispersing agent in one or more doses may be added to the suspension after at least some of the evaporative dewatering in step (d).
The suspension may following the said working step (e) and following further optional dispersing agent addition be dewatered to give a further differential increase in the solids content of the suspension by thermal evaporation under reduced pressure. The further differential increase in solids content after working after some evaporative dewatering may be at least 5% by weight.
The suspension at its maximum concentration following the evaporative dewatering step(s) may be treated by further working and/or dispersing agent addition prior to storage or use.
The suspension obtained as a product of the method of the invention may be a fluid and Theologically stable aqueous suspension of the carbonate having a solids concentration of at least 65% by weight, e.g. at least 68% by weight, in many cases from 70% to 80% by weight.
Preferably, the or each addition of the dispersing agent is accompanied by stirring of the suspension either whilst the dispersing agent is added or after the addition of the dispersing agent. The said working applied may be provided by the said stirring, e.g., by applying the stirring in a blunger, or may be applied separately from and additional to the said stirring.
The working may be applied in at least two steps, namely after some of the said evaporative dewatering followed by further evaporative dewatering and then again later after the further evaporative dewatering. In addition, working may optionally be applied prior to the evaporative dewatering. The working in each in each case may be applied by a mechanical device in one of the ways described later.
The particulate carbonate prepared in step (a) may comprise calcium carbonate although it will be apparent to those skilled in the art that the method of the invention may also be applied to other carbonates such as magnesium carbonate and barium carbonate.
The carbonate in step (a) may be prepared from a natural source, e.g. chalk, marble or limestone, by known grinding procedures. Alternatively, the carbonate may be synthesised. The invention is particularly suitable to treat calcium carbonate which has been prepared by carbonation of lime in a dilute aqueous medium, e.g. by use of carbon dioxide. The carbonate produced in this manner is known as precipitated calcium carbonate.
Where the carbonate prepared in step (a) is a precipitated calcium carbonate, it may be in the calcite, aragonite or vaterite form or a mixture of two or more of these forms or different shapes thereof. The conditions required to produced these various crystal forms and various shapes thereof are well known to those skilled in the art. The vaterite form is thermodynamically unstable at normal temperatures. The aragonite form is metastable under normal ambient condition, but converts to calcite at high temperatures. The calcite form is the most stable, and can exist in several different crystal shapes of which the rhombohedral and scalenohedral shapes are probably the most important.
In step (b) of the method of the invention, the dewatering may be effected by one or more known mechanical devices, e.g. by means of a plate filter press, a vacuum filter, a membrane cross-flow filter or a centrifuge or two or more
Mortimer Mark Daniel
Purdey John Anthony
Skuse David Robert
Thrale Deborah Susan
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Imerys Minerals Limited
Lovering Richard D.
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