SUITABLE REAGENT FOR THE TREATMENT OF HIGH-SULPHATE WATERS

Abstract

The present invention relates to the manufacture of a chemical reagent whose principal active constituent comprises hydrated metastable forms of CAH.sub.10 and C.sub.2AH.sub.8 in aqueous suspension, and for use of the reagent within other processes where an aqueous suspension of precipitated calcium aluminate is required. The present invention further relates to the use of one or more particle segregation stages to assist the chemical reaction processes that are involved.

Claims

1. A process for the manufacture of a chemical reagent whose principal active constituents comprise hydrated metastable forms of chemical compounds having the generally accepted chemical formulae Ca(Al(OH).sub.4).sub.2.6H.sub.2O and Ca.sub.2(Al(OH.sub.4(OH).sub.2.3H.sub.2O (or CAH.sub.10 and C.sub.2AH.sub.8 when using Cement Industry notation) in aqueous suspension, the process comprising: a reaction stage comprising one or more reaction vessels, at least one, of which is operating at a pH in excess of pH 11.5, the reaction stage including a mixing means and further comprising; addition of an aqueous phase, addition of lime and/or an alternative calcium and hydroxide containing reagent, addition of pieces of aluminium in metal form, the mixing means associated with each reaction vessel being arranged to maintain each item of a solid material that is added to the vessel in continuous, semi-continuous or frequent abrasive contact with respect to inner surfaces of the vessel, materials that may be present within the vessel and with materials that may subsequently become present within the vessel; and a particle separation stage including a separation means, or an assembly of separation means which exploit(s) differences within particle settling velocities for achieving separation of particles from the reaction stage into a first product and a second product, the first product comprising more than 50% of CAH.sub.10 particles and of C.sub.2AH.sub.8 particles that are precipitated within the reaction stage and of more than 50% of particles with a lower settling velocity within an accompanying aqueous phase than that of the precipitated CAH.sub.10 particles and of the precipitated C.sub.2AH.sub.8 particles within the accompanying aqueous phase, the second product comprising more than 50% of the particles with a higher settling velocity within the accompanying aqueous phase than that of the precipitated CAH.sub.10 particles and of the precipitated C.sub.2AH.sub.8 particles within the accompanying aqueous phase, between zero and 100% of the second product being returned to the reaction stage, and between 5 and 100% of the first product being outputted from the process as the chemical reagent.

2. The process of claim 1, wherein the reaction stage further comprises the addition of particles of abrasive material(s).

3. The process of claim 1, wherein the reaction stage includes oxidising means for promoting an oxidising environment.

4. The process of claim 1, wherein the separation means includes one or more devices selected from hydrocyclones, centrifuges, gravity separation, elutriation, screening, filtration and screw classification, or any combination of such devices.

5. The process of claim 1, wherein adjunct materials selected from iron hydroxides, hydrated iron oxides, hydroxides of other metals which are below aluminium within the electrochemical series, and hydrated oxides of other metals which are below aluminium within the electrochemical series, in any combination, are added to the reaction stage.

6. The process of claim 1 wherein between zero and 100% of the aqueous phase that is added to the reaction stage is sourced from the reactor(s) within which the chemical reagent is used or from a subsequent neutralisation stage that follows the reactor(s) within which the chemical reagent is used.

7. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will now be described in more detail, by way of example only, with reference to the accompanying FIGURE in which:

[0067] FIG. 1 shows a diagram illustrating a preferred embodiment of the process of the present invention.

[0068] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of certain embodiments of the present invention by way of the following non-limiting example.

EXAMPLE: SUITABLE REAGENT FOR ETTRINGITE BASED TREATMENT OF SULPHATE-RICH WATERS

[0069] With reference to FIG. 1, the extent of the reaction stage equipment is shown diagrammatically by the dotted boundary 12. Within that boundary there is the reaction vessel or vessels 2 and the particle segregation stage 8. The particle segregation stage would be a segregation technology, a segregation device or an assembly of segregation devices selected from the list comprising hydrocyclones, centrifuges, gravity separation, elutriation, screening, filtration and screw classification,

[0070] The necessary aqueous phase for the reaction stage 12 is shown as entering the reaction vessel(s) 2 at arrow 1. Arrows 3, 4 and 5 indicate the inputs for the particles of aluminium and/or other Additional Sources of aluminium at 3, the lime and/or the calcium and hydroxide containing material at 4 and the abrasive at 5. The dotted arrow 6 indicates the optional addition of adjunct materials which could consist of one or more of additional abrasive material, particles of coal, particles of coke, particles of other forms of elemental carbon, particles of iron, particles of steel, ferric hydroxide, hydrated ferric oxide, ferrous hydroxide, and hydroxides and hydrated oxides of metals which are below aluminium within the electrochemical series.

[0071] The arrow 7 indicates the input flow of mixed material from the reaction vessel(s) 2 into the particle segregation stage 8. Arrow 9 indicates the return flow from the particle segregation stage of the segregated particles which include those particles which have a higher settling velocity than the calcium aluminate particles which are produced within the reaction vessel(s). Arrow 10 indicates the forward flow of particles which include calcium aluminate particles which are produced within the reaction vessel(s) and those particles which have a lower settling velocity than the calcium aluminate particles.

[0072] Arrow 11 indicate the occasional purge flows that may become necessary for purging abrasive particles which have lost their abrasiveness and for any other metallic or other impurities that may accumulate within the process as a result of contaminants, etc. within the feed materials to the process.

[0073] In some situations it may be appropriate to have an additional particle segregation stage 13 for creating an improved removal efficiency in respect of the metals, the abrasive(s) and the other reaction assisting particles from the aluminium reagent 14 and for returning them to the reaction vessel(s) 2 via stream 15. As this is an optional feature within the process it has been shown using dotted lines. Also shown in dotted lines is a purge stream 16 which would complement the occasional function of stream 11.

[0074] Vessel 17 indicates a suitable buffer storage unit for the aluminium reagent 14. This buffer storage unit is an optional but preferred feature and would be equipped with a suitable agitation arrangement to maintain in suspension the suspended particles within the reagent 14. The reagent would be forwarded to the ettringite based sulphate removal process or to wherever else it is to be used via arrow 18 on an as required basis using one or more of the many suitable feeding and metering arrangements as are well known to a skilled practitioner.

[0075] As noted above, the aqueous input 1 to the process would be preferably derived from, or downstream from the reaction stage(s) within which the aluminium reagent is used. In this way hydrated oxides and hydroxides of metals that may be present within the reaction stage where the aluminium reagent is used or which may be precipitated within subsequent neutralisation stages could be included within the input to the reaction vessel(s) 2. In particular, it is frequently the case that aluminium hydroxide will be precipitated within the neutralisation stage that would normally follow an ettringite based sulphate removal stage and the resultant concentrate of precipitated particles from a clarification stage following this neutralisation stage will enable this aluminium to be recovered into the aluminium reagent.

[0076] Also, as noted above, it is beneficial to raise the temperature within the reaction vessel(s) 2, subject to an upper temperature limit of about 50 C. Preferentially this would be achieved by preheating stream 1 before it is added to the process. However, additional heat could be added to stream 9, or by the direct introduction of steam into the reaction vessel(s) or by a heat transfer process within or through the walls of the reaction vessel(s) or by other means. From the perspective of potential scale formation, stream 9 represents the location where scale formation as a result of heat input is least likely to occur providing the heat transfer surface temperatures are maintained below about 55 C.

[0077] It should be noted that the slow conversion process of calcium aluminate from the metastable CAH.sub.10 and C.sub.2AH.sub.8 forms into the C.sub.3AH.sub.6 form is also assisted by an increase in temperature. A compromise must therefore be selected between increasing the reaction rates within reaction vessel(s) 2 and minimising the conversion of the metastable calcium aluminate into the considerably less soluble C.sub.3AH.sub.6 form within the subsequent stages 8, 13 and 17. The selection of the optional technology that is used within stages 8 and 13 (if present) should therefore be biased towards technologies which involve a minimum residence time within stages 8 and 13. Similarly, the sizing of the buffer storage facility 17 should be on the basis of minimising the inventory that is retained within 17 subject to process control and stability considerations within the process(es) within which the aluminium reagent is to be used.

[0078] Preferably, within the reaction vessel(s) 2, the environment within the aqueous phase needs to be maintained as an oxidising environment. Subject to the design features of the reaction vessel(s), this can be achieved preferentially by maintaining a plentiful supply of ambient air within/through the vessel(s). In addition or alternatively it may be appropriate to add a compressed air sparging arrangement or a suitable oxidising agent such as hydrogen peroxide. Preferentially, if a suitable oxidising agent is to be added, it should be added to Stream 1 before Stream 1 enters the reaction vessel(s) 2.

[0079] The reaction vessel(s) 2 can be selected from a range of well-established industrial devices that will be known to a skilled practitioner. Suitable equipment could be selected from the list comprising pan mixers, Muller mixers, raked pits, rotating inclined pans, rotating drum mixers, plough share mixers, ribbon mixers, cone mixers, ball mills, autogenous mills and other similarly capable equipment.

[0080] Typically, Stream 7 will consist of a slurry with a suspended solids content within the range of 1 to 30% by weight and preferably within the range of 3 to 15% by weight. However, the process would still be effective when the suspended solids content is outside the above referred typical range.