Process for flocculating suspensions

Details Process for flocculating suspensions Process for flocculating suspensions

2002-01-09

2004-10-19

Hruskoci, Peter A. (Department: 1724)

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

C210S734000, C210S738000, C210S928000, C252S180000, C422S901000, C524S922000

Reexamination Certificate

active

06805803

ABSTRACT:

This invention relates to processes of flocculating aqueous suspensions in order to effect separation of solids from said suspension.
It is well known to apply polymeric flocculants to aqueous suspensions in order to separate solids from the suspension. For instance it is common practice to flocculate and then dewater suspensions containing either suspended solid organic material or mineral solids. For instance it is common practice to flocculate sludges such as sewage sludge, waste waters, textile industry effluents, red mud from the Bayer Alumina process and suspensions of coal tailings etc. Flocculants are also commonly used in paper-making processes by addition of polymeric flocculants to the cellulosic suspension. Flocculation is usually achieved by mixing into the suspension polymeric flocculant, allowing the suspended particles to flocculate and then dewatering the flocculated suspension. In papermaking this removal of water from the cellulosic suspension is often referred to as draining.
High molecular weight polymeric flocculants are commonly used for this purpose. High molecular weight flocculants may be cationic, anionic, nonionic or amphoteric in nature. The choice of polymeric flocculant will largely depend upon the susbstrate, which is being treated. For instance it is common practice to use high molecular weight cationic flocculants to treat aqueous suspensions comprising suspended organic material, for instance sewage sludge. In paper-making it is known to use either cationic, nonionic, anionic or amphoteric flocculants. Flocculation of mineral suspensions is frequently effected by use of anionic flocculants.
It is also known to use two different polymeric flocculants in the same process. The flocculants may have the same charge (co-ionic). For instance in commercial practice in the dewatering sewage sludge these may be co-ionic. In other processes it is known to apply two polymers of opposite charge (counter-ionic). Where two polymeric flocculants are applied to an aqueous suspension they may be added to simultaneously or more usually sequentially.
It is standard practice to apply polymers as aqueous solutions to flocculate suspensions containing suspended organic material. Generally the solutions of polymers are relatively dilute, for instance below 0.5%, often below 0.3% and usually 0.1% to below 0.2% by weight.
Polymers are usually provided as a solid particulate product or as a reverse phase dispersion or emulsion. It is standard practice to dissolve the polymer into water by dispersing the polymer particles in a flowing stream of water in the case of the solid particulate product or in the case of the emulsion or dispersion, inversion into water, by use of activator surfactants. The polymer solution thus formed is frequently at a concentration above 0.3%, often in the range 0.4% to 1% and usually about 0.5%. This more concentrated solution of polymer may be too concentrated to add directly to the suspension in many instances, since received wisdom suggests that there would be inadequate distribution of the flocculant throughout the suspension and as a consequence the flocculation process would be impaired.
It is therefore common practice to first of all provide a more concentrated solution of polymer and then dilute the polymer solution prior to application. Often the diluted solution will have a concentration of below 0.2%, for instance within the range 0.05 to 0.2% by weight and frequently between 0.1 and 0.2% by weight. This dilute solution of polymer is normally metered directly into the suspension prior to the dewatering stage.
There is a desire to improve the efficiency of the flocculation processes, to either bring about an increased dewatering effect, such as higher cake solids or in the alternative achieve a constant acceptable level of dewatering efficiency but using a lower dose of flocculant. This is true in a variety of flocculation processes, including dewatering of sewage sludge, slurries of coal tailings, red mud and in papermaking.
It would therefore be desirable to provide an improved method of flocculating and dewatering aqueous suspensions of solids, in particular to provide increased dryness of the dewatered solids for an equivalent dose of flocculant or to provide the same degree of dryness of dewatered solids but using a reduced dose of flocculant. It would also be desirable to provide a process which provides faster dewatering.
The invention relates to a process of flocculating and dewatering an aqueous suspension of suspended solids comprising, introducing into the suspension,
(a) a concentrated polymer solution and,
(b) a dilute polymer solution,
characterised in that the concentrated and dilute polymer solutions are introduced into the substrate substantially simultaneously.
The concentrated and dilute solutions may be metered directly into the suspension as separate solutions. By substantially simultaneously the two solutions should be added at approximately the same dosing point. Where the concentrated and dilute solutions are added to the suspension separately, they may be added in either order. For instance if the dilute solution is added first the concentrated polymer may be added after flocculation has commenced but should be added before the dewatering stage and before any high shear stage, such as pumping or screening stages. Alternatively, it may be desirable to add the dilute polymer solution after the addition of the concentrated polymer solution. When the dilute and concentrated polymer solutions are added separately it may be appropriate to allow or apply some degree mixing between the dosing stages in order to allow the first polymer dose to become distributed throughout the suspension solids. This mixing may for instance include allowing the treated suspension to pass some distance along a flow line which optionally contains bends, baffles, constrictions or other features which induce gentle mixing.
Preferably the concentrated and dilute polymer solutions are introduced simultaneously.
More preferably the concentrated and dilute polymer solutions are introduced into the suspension as an aqueous composition comprising a dilute aqueous solution of polymer and a concentrated solution of polymer. The aqueous composition should comprise both the dilute and the concentrated polymer solutions as discrete components. Thus it is desired that the dilute solution and concentrated solution exist as substantially discrete components of the aqueous composition.
The aqueous composition preferably comprises the dilute aqueous solution of polymer in an amount of from 20 to 99%, based on weight of polymer, and the concentrated polymer solution in an amount of from 1 to 80%, based on weight of polymer. For some applications, such as for rotary vacuum filtration coal tailings slurries it may be appropriate to use a ratio of concentrated solution to dilute polymer solution of around 75:25. However in most other applications the ratio of concentrated polymer solution to dilute polymer solution would generally in the range 1:99 to 40:60.
The aqueous composition comprising concentrated and dilute solutions may be of any significantly different concentrations provided that the respective concentrations are not substantially the same such that the two solutions would immediately form a homogenous single solution. Preferably the concentrated solution should be at least twice the concentration of the diluted solution. More preferably the concentrated solution should be at least 4 or 5 times the concentration of the dilute aqueous solution.
The dilute aqueous solution of polymer desirably has a concentration of polymer of below 0.5%, preferably below 0.3% by weight. More preferably the concentration of the dilute solution is in the range 0.05 to 0.2%, most preferably around 0.1% by weight.
According to the invention the polymer dissolved in the dilute aqueous polymer solution may be either cationic, anionic or non-ionic.
The concentrated aqueous solution component according to the invention desirably has a concentration of polymer above 0.3%

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