Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1994-06-22
1995-09-26
Hruskoci, Peter A.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210714, 210725, 210727, C02F 156
Patent
active
054532060
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for removing silica from aqueous liquors.
It is known that soluble and colloidal silica present in water that is used in cooling water circuits in gas or coal fired power stations or other applications is undesirable since it tends to form extremely hard deposits that progressively reduce the heat transfer efficiency of the cooling water circuits.
There are a number of known chemical and physical means for removing the silica deposits. However, all these known means have disadvantages. For example, one physical means comprises projecting a scraper bullet through the tubing in cooling water circuits to scrape away the deposits. The disadvantage of this technique is that there is an unacceptable risk of damage to the cooling water circuits.
An object of the present invention is to provide a process for removing soluble and colloidal silica from aqueous liquors to minimise the extent of the silica deposits.
According to the present invention there is provided a process for removing silica in dissolved and/or colloidal form from an aqueous liquor comprising: precipitant/adsorbent; precipitated/adsorbed silica and any other suspended solids in the liquor and the flocculent; and
The term "precipitant/adsorbent" is understood herein to mean any element or compound in soluble or insoluble form that is capable by mechanisms of precipitation or adsorption of changing dissolved and/or colloidal silica into a state that can be separated more readily from the liquor.
It is preferred that step (a) comprises adjusting the pH of the liquor to promote precipitation/adsorption.
It is particularly preferred that step (a) comprises adjusting the pH to be equal to or greater than 8.0.
It is more particularly preferred that step (a) comprises adjusting the pH to be equal or greater than 8.5.
Typically, step (a) comprises adjusting the pH to be equal or greater than 9.0.
It is preferred that the process further comprises the step of adding an inert particulate carrier prior to or at the same time as the step (b) of adding the flocculent.
It is particularly preferred that the inert particulate carrier is added prior to the addition of the flocculent.
It is preferred that the process further comprises, after step (c), agitating the floccs to break up the floccs and separating the inert particulate carrier for recycling in the process.
It is preferred that the precipitant/adsorbent comprises a compound containing one or more of aluminium, magnesium, and iron. It is noted that in some situations the precipitant/adsorbent may already be in the liquor and the pH adjustment is required to promote precipitation/adsorption of the dissolved or colloidal silica with/onto the precipitant/adsorbent. It is also noted that in other situations it may be necessary to add the precipitant/adsorbent to the liquor, with or without pH adjustment.
It is particularly preferred that the precipitant/adsorbent is a soluble aluminium salt such as sodium aluminate, aluminium sulphate or aluminium chloride.
It is particularly preferred in situations where the precipitant/adsorbent is a soluble aluminium salt that the process comprises, after step (c), lowering the pH of the liquor to 7.5 or less to precipitate any soluble aluminium retained in the liquor.
The term "liquor" is understood herein to include aqueous and non-aqueous liquors.
The term "inert" as used herein in relation to "particulate carrier" is understood herein to mean that the particulate carrier is not substantially attacked by the liquor. In other words, the term "inert" means that the particulate carrier exhibits both suitable chemical and physical stability in the liquors.
It is preferred that the inert particulate carrier is selected from the group consisting of sand, alumina, garnet, magnetite, hematite, ilmenite, and calcite.
The term "suspended solids" is understood herein to include organic and inorganic material.
It is preferred that the flocculent is a polyelectrolyte flocculent.
The term "polyelectrolyte flocculent" as used herein is under
REFERENCES:
patent: 3142638 (1964-07-01), Blaisdell
patent: 3377274 (1968-04-01), Burke et al.
patent: 3388060 (1968-06-01), Clark
patent: 4279756 (1981-07-01), Weiss et al.
patent: 4783265 (1988-11-01), Timmons
patent: 4882064 (1989-11-01), Dixon et al.
patent: 4997573 (1991-03-01), Browne
Hruskoci Peter A.
Modern Environmental Service Trust
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