GB2035816A

GB2035816A - Flocculation

Info

Publication number: GB2035816A
Application number: GB7846647A
Authority: GB
Original assignee: RYAZAN Z PROIZV I OBRABOT TSVE; SP KONSTRUKTBJURO TYAZHEL TSVE
Current assignee: RYAZAN Z PROIZV I OBRABOT TSVE; SP KONSTRUKTBJURO TYAZHEL TSVE
Priority date: 1978-11-30
Filing date: 1978-11-30
Publication date: 1980-06-25
Also published as: GB2035816B

Abstract

Flocculation suitable for separating a suspension into a solids fraction and a liquid fraction. Mixing of flocculating agent and suspension is achieved by injecting these components into a venturi tube through which a gas is passing at high speed.

Description

SPECIFICATION Flocculation The present invention relates to a method of flocculating particles in a suspension, which can be used to separate the suspension. This method of separating solid and liquid phases is particularly useful in, for example, the chemical and metallurgical industries.

Use of flocculating agents in liquid-solid separations is known, and the suspension and flocculating agent have generally been mixed simply by mechanical stirring. This method, however, does not result in very fast settling of solid particles and a large amount of liquid tends to become trapped in the settled fraction. We have now discovered a method of mixing a flocculating agent with a suspension that can overcome these disadvantages.

Thus, the invention consists in a method of flocculating particles of a suspension, which comprises injecting the suspension, preferably in pre-atomized form, and a flocculating agent (preferably pre-sprayed), such as polyacrylamide, into a gas stream. Preferably, the suspension is injected by spraying it through a sprayer.

We prefer that the gas, which is conveniently air, passes through a venturi tube, and the flocculating agent and the suspension are injected into the venturi. In order to achieve satisfactory mixing of the suspension and the flocculating agent it is necessary that the gasliquid stream that results has the correct flow characteristics. The characteristics of this stream will depend on the rate of injection of suspension and of flocculating agent and on the rate of flow of the gas, and suitable values can easily be determined in practice. However, we have found that good results are obtained when the Reynolds No. (Re No.) of the resulting stream is from 100,000 to 150,000 since the suspension and the flocculating agent are mixed completely very quickly without the formation of flocculi and this intensifies the process and enhances its effectiveness.When the Reynolds No. is outside this range separation of the suspension may be reduced considerably.

Treatment of a suspension may be carried out as follows: a high-velocity stream of air or other suitable gas is fed to the mouth of a veturi pipe. A pre-atomized suspension and a pre-sprayed flocculating agent are then injected into this stream of air. Any sprayer suitable for spraying flocculating agents and suspensions may be used for this purpose. Best results are obtined when the suspension and the flocculating agent are injected separately and co-currently with the air stream. Also, better results are obtained when the suspension, as well as the flocculating agent, is injected pre-sprayed.

The exact arrangement of the two injectors is not critical, but we prefer that the sprayer or other injector for the flocculating agent is situated above the sprayer that disperses the suspension.

The gas-liquid stream that results may then be transferred to another apparatus where coagulation and settling are allowed to occur. Therefore, the invention also consists in a process for separating a suspension, which comprises subjecting the suspension to the method of flocculation of the invention, and allowing the resulting mixture of flocculating agent and suspension to settle.

The method of the invention has several advantages, for example: The rate of settling of solid particles can easily be from 5 to 10 times greater than that achieved by conventional methods; The quality of the thickened fraction is very high since it contains only a small amount of liquid. This thickened fraction may have a liquid:solid ratio of about 1.1:1, compared to 3:1 for most known methods; The amount of solids suspended in the liquid fraction can be reduced by a factor of from 3 to 5; The amount of flocculating agent required by the method of the invention can be from 25 to 30% less than that required for known methods and The method of the invention is simpler and consumes less power than does filtration.

The method of the invention is simple to carry out and it is easily automated also, special equipment and techniques are not required to control the process.

The invention is further illustrated by the following example.

Example Here, separation was carried out on a suspension obtained during sulphuric acid leaching of dusts containing tin, zinc, lead and volatilized compounds. The suspension has a liquid:solid ratio of 5:1 and contained 120 g/l of ZnSO4 (based on Zn) and 5 g/l of H2SO4 in its liquid phase. The solid phase consisted of lead sulphates, tin hydrates and zinc stannates.

The suspension and a pre-atomized flocculating agent were mixed by spraying them into a venturi pipe. The flocculating agent was polyacrylamide, and it was consumed at a rate of 400 g/t of solids. The air-liquid stream turbulence had a Reynolds No. of 120,000.

After the mixing, the suspension was settled in a cylinder of height 2 m. During the first five minutes of settling, the rate of clarification of the mixture was 14 m/h, and after thirty minutes of settling the liquid:solid ratio was 1.1:1 and the amount of solid material in the clarified portion was 1.9 g/l.

This procedure was repeated for air-liquid streams having various Reynolds Nos. The rates of clarification (m/h), the liquid:solid ratios of the thickened fractions, and the amount of suspended matter in the clarified fractions (g/l) for these various Reynolds Nos. were as follows.

Clarification Liquid 8 Solid Suspended Rate Ratio Matter Reynolds Number 80,000 9 1.8:1 6.0 100,000 13 1.4:1 2.7 150,000 13.5 1.3:1 2.5 170,000 10 1.7:1 5.3 Comparative Example This comparative example was applied to a suspension produced during sulphuric acid leaching of a dust bearing tin, zinc, lead and volatilized matter. The suspension had a liquid:solid ratio of 5:1 and contained 120 g/l ZnSO4 (based on Zn) and 5 g/l H2SO4 in its liquid phase. The solid phase consisted of lead sulphates, tin hydrates and zinc stannates.

The suspension was treated with polyacrylamide as flocculating agent at an amount of 400 g/t of solids. The mixture was mixed for six s in an agitator by mechanical stirring, giving a Reynolds No. of 10,000.

After mixing the suspension was settled in a cylinder of height 2 m. During the first five minutes the rate of clarification was 1.57 m/h, and after thirty minutes of settling the liquid:solid ratio in the thickened fraction was 2.6:1, and the amount of suspended matter in the clarified fraction was 4.6 g/l.

Claims

1. A method of flocculating particles of a suspension, which comprises injecting the suspension and a flocculating agent into a gas stream.

2. A method according to Claim 1, in which a pre-atomized suspension is injected.

3. A method according to Claim 1 or Claim 2, in which the suspension is injected by spraying.

4. A method according to any one of the preceding Claims, in which a pre-sprayed flocculating agent is injected.

5. A method according to any one of the preceding Claims, in which the gas is air.

6. A method according to any one of the preceding Claims, in which the gas passes through a a venturi tube, and the flocculating agent and the suspension are injected into the venturi.

7. A method according to any one of the preceding Claims, in which the Reynolds No. of the air stream after injection of flocculating agent and suspension is from 100,000 to 150,000.

8. A method according to any one of the preceding Claims, in which the flocculating agent and the suspension are injected through different injectors.

9. A method according to Claim 8, in which the flocculating agent is injected into the gas stream at a point above that at which the suspension is injected.

10. A method according to any one of the preceding Claims, in which the flocculating agent is polyacrylamide.

11. A method according to Claim 1, substantially as herein described with reference to the foregoing example.

12. A process for separating a suspension, which comprises flocculating the suspension by a method according to any one of the preceding Claims, and allowing the resulting mixture of flocculating agent and suspension to settle.