JPH0137979B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides solid matter, suspended matter, aluminum,
The present invention relates to a method for pre-treating zinc-containing wastewater containing impurities such as iron when concentrating the zinc component using a reverse osmosis membrane method or the like.

Acidic wastewater containing zinc as its main component is discharged from factories that handle zinc, such as mines and the Metsuki factory. These wastewaters usually contain suspended solids such as sand and carbon particles, and trace amounts of iron, aluminum, and other heavy metal ions.

Conventionally, these wastewaters were often separated by precipitation together with other metal hydroxides at pH 10, where the solubility of zinc hydroxide was lowest, and the sludge was dehydrated and then disposed of. In this treatment method, a large amount of alkaline agent is first required to raise the pH of wastewater with a pH of 4 or lower to 10. Furthermore, when zinc, iron, aluminum, and other metal ions become hydroxides, a chemical amount of Theoretically, a large amount of alkaline agent is consumed, and the presence of coexisting impurities hinders the recovery and recycling of expensive zinc. Calcium hydroxide, carbide slag, or sodium hydroxide is commonly used as an alkaline agent in conventional methods because it is cheap and easily available. Carbide slag or calcium hydroxide are often used because of their excellent dehydration properties. However, in the case of calcium hydroxide, the insoluble calcium hydroxide added for pH adjustment mixes into the sludge, resulting in an increase in the amount of sludge generated from zinc-containing waste. In addition, when sodium hydroxide is used, the amount of sludge generated is small, but since it is less effective as a flocculation and dewatering aid like calcium hydroxide, the generated sludge is made up of metal hydroxide. Most of the substances are precipitated and have poor dehydration properties. For this reason, the sedimentation separation tank requires a large water area and the dewatering speed is slow, resulting in the drawback that the equipment for excessive dewatering of sludge becomes large.

As mentioned above, when the treatment is carried out by the conventional method, a large amount of alkaline agent is required, and the disposal cost of the generated sludge and the cost of treatment equipment are increased, which are serious drawbacks.

The purpose of the present invention is to solve these drawbacks of conventional methods by reducing the amount of alkaline agents used and at the same time reducing the amount of sludge generated, thereby reducing chemical costs, sludge disposal costs, and sludge dewatering equipment costs. The goal is to provide the following.

Recently, metal ions in wastewater have been increasingly collected and reused for environmental reasons and resource reuse. Generally speaking, methods for concentrating metal ions include evaporation, freezing concentration, and ion exchange. , electrodialysis, extraction, and reverse osmosis, but recently reverse osmosis membranes have been used because they require less energy, only pressurize the liquid, do not require the use of other chemicals, and are easy to operate. There is interest in the method of enrichment by method.

The most important problem with reverse osmosis concentration is membrane clogging. That is, contamination is due to adhesion of suspended substances in the liquid to the membrane surface and scale precipitated from dissolved components. To prevent contamination of these membrane surfaces, cleaning by dissolving clogging components with chemicals, flushing with water, and physical cleaning with sponges or air bubbles are used, but none of these methods are perfect, and membrane contamination can cause If the amount of permeated water decreases extremely,
This situation necessitates membrane replacement, and it is important to prevent membrane surface contamination from occurring.

In water treatment, when removing suspended solids, inorganic flocculants such as ferrous sulfate, ferric polychloride, aluminum sulfate, and polyaluminum chloride are added, PH is adjusted, and metal hydroxide is added. It is customary to form a polymer of a substance, coagulate it with suspended substances, and perform solid-liquid separation.

Another object of the present invention is to coagulate impurities such as suspended solids by simply adjusting the pH without adding any coagulant, by skillfully utilizing the properties of coexisting metal ions contained in zinc wastewater. It is an object of the present invention to provide a method for separating solid and liquid, and furthermore, for producing a scale-free and condensable liquid at low cost.

To explain the present invention in detail with reference to the drawings, in Fig. 1, raw water with a pH of 4.0 or less is introduced from an inflow pipe 1 into a stirring tank 2, and is concentrated with an alkaline agent such as sodium hydroxide. The pH is adjusted in advance to within the range of 5.5 to 6.5, at which only the hydroxides of aluminum and iron are precipitated, and the hydroxides of zinc do not precipitate. It can be aggregated without adding any agent. This is because iron ions and aluminum ions, which are coexisting impurities in the raw zinc wastewater, become hydroxide polymers and precipitate, which easily forms suspended solids and scale in the raw water. This is to coprecipitate silica and some dissolved substances. In some cases, the stirring tank 2 may be divided into two tanks: a rapid stirring tank for uniformly mixing the raw water and the alkaline agent, and a slow stirring tank for aging the flocs.

The raw water containing these flocs flows into the settling tank 5 through the conduit 4, where the flocs settle and are separated from the supernatant water. In this explanation,
Although the sedimentation tank 5 shown in the figure was used to perform sedimentation separation, separation can also be performed using other high-speed flocculation and sedimentation tanks. Supernatant water from settling tank 5 is introduced via conduit 6 into intermediate storage tank 7 .

This supernatant liquid contains minute solids that could not be separated in the sedimentation tank, and must be further clarified by filtration. Therefore, the supernatant water is transferred to an over-pump 9 connected to the intermediate storage tank 7 via a conduit 8.
is introduced into the overtank 11 from the conduit 10 by
It is filtered through the material 12 filled in the filter tank 11. The material 12 is usually made of sand or sand and anthracite, but if the raw water contains manganese ions, manganese sand may be used instead of sand in order to oxidize and adsorb the manganese ions. In addition, if the supernatant water from the sedimentation tank 5 can be directly introduced into the supernatant tank through the conduit 6 by using a gravity-type supertank as the supertank 11, the intermediate storage tank 7, the liquid pump 9, and the conduit pipe may be used. It goes without saying that 8 and 10 can be omitted. Also, although omitted in this figure,
The overflow tank 11 is used for periodic backwashing with overflowing water.

In this way, iron ions and aluminum ions that coexist in zinc wastewater are used with a small amount of alkaline agent, and during concentration using the reverse osmosis membrane method, suspended solids that tend to adhere to the membrane surface, silica, and other substances precipitate in the liquid. iron ion,
Aluminum ions can be removed very economically and easily.

The excess water stored in the excess water tank 14 is transferred to the conduit 15
Through the booster pump 16, the conduit 17
The water is then supplied to a precision filter 18 used for membrane protection. An acid such as hydrochloric acid is added to this conduit 17 to further readjust the pH to 3.5 to 5.5. This is a consideration to prevent the formation of zinc and other metal hydroxide layers and the precipitation of scales such as calcium carbonate on the membrane surface when zinc is concentrated. From the viewpoint of corrosion resistance of the equipment, it is desirable that the pH value at this time is neutral, but depending on the final concentrated concentration of zinc,
The pH value is set at which zinc hydroxide does not precipitate.

With the above steps, the pretreatment for concentration by reverse osmosis is completed, and after that, the pressure is increased to about 20 to 60 kg/cm ² using the pressure pump 20 connected to the precision filter 18 through the conduit 19, and the conduit 21 is passed through the conduit 21. The water is then supplied to a module 22 containing a reverse osmosis 23, where it is divided into concentrated water 25 and membrane permeated water 24. Concentrated water 25 is then used for reuse and recovery of zinc. Furthermore, the permeated water can be reused as water if the water quality is good.

Note that the sludge that has been sedimented and separated in the settling tank 5 is transferred to the conduit 2.
6 to the dehydration equipment 27, and after dehydration treatment,
It is discharged as a dehydrated cake 28. Further, the dehydrated liquid is mixed with the raw water in the conduit 1 through the conduit 29 and reprocessed.

The sludge generated by the present invention contains impurities other than zinc, especially fine sand, carbonaceous suspended solids, iron ions, and aluminum ions, so the higher the ratio of zinc to the impurities in wastewater, the higher the concentration of zinc. Compared to the conventional PH10 method, which treats the entire amount including zinc ions as sludge, the sludge generation rate is lower, the consumption of alkaline agents is reduced, the sludge treatment cost is reduced, and the concentration of zinc is reduced. , the recovery benefits will also be greater.

In addition, as another example, if the iron ions contained in the raw water contain divalent iron ions, it is necessary to oxidize, and in this case, sodium hypochlorite or chlorine is added to the stirring tank 2. By adding an oxidizing agent such as, trivalent iron ions can be easily converted into trivalent iron ions, and iron ions can be completely separated in the precipitation tank 5.

Additionally, if the raw water contains calcium ions, calcium-based scale may be generated during concentration, so adding a chelating agent such as polyphosphoric acid to the conduit 17 as an ion sequestering agent for calcium ions will reduce the scale. This can be prevented from occurring.

As described above, the present invention enables zinc wastewater containing solids, suspended solids, aluminum ions, and iron ions to be treated with zinc using a reverse osmosis membrane by simply adjusting the pH to a specific pH in advance. Since the main component, zinc, does not turn into sludge, the amount of PH adjusting agent is small, and the amount of sludge generated is low, so the processing cost is low. They were able to establish a method that reduces the amount of impurities, allows recycling, and has high added value.
Furthermore, the higher the concentration of zinc ions in wastewater, the higher the treatment cost in conventional methods, but in the present invention, the merits of concentration of zinc ions are increased, making it a very useful method.

Next, the present invention will be explained based on an experimental example.

Experimental Example 1 When zinc wastewater discharged from a certain factory was subjected to PH titration with sodium hydroxide, the results shown in Figure 2 were obtained.

This wastewater contains 1500ppm zinc ions, 10ppm aluminum ions, 21ppm iron ions, 20ppm soluble silica, 11ppm suspended solids, and the pH is
It was 2.5.

From Figure 2, 350 ppm of sodium hydroxide was required to adjust the pH of this wastewater to 6.5, and 1650 ppm to adjust the pH to 10.0.

From this result, it was found that in the present invention, the amount of alkaline agent required is about 1/5 compared to the conventional method.

Experimental example 2 PH the same raw water as in experimental example 1 with sodium hydroxide.
6.5, precipitate and separate the precipitated flocs, pass the supernatant water through a filter tank filled with anthracite and sand, and then adjust the pH to 4.0 with hydrochloric acid.
1400ppm, aluminum 0.1ppm or less, iron 0.1ppm
A very clear liquid was obtained with 7.0 ppm of soluble silica and 0.1 ppm of suspended solids.

This pre-treatment liquid was applied to a reverse osmosis device with a modularized reverse osmosis membrane with a membrane area of 0.7 m ² at a pressure of 50 Kg/cm ^{2 .}
As a result of concentration, the average amount of water permeated through the membrane was 55/m ²
We were able to concentrate the zinc concentration to 11,000ppm using Hr. The recovery rate of zinc at this time was 78.5% based on the amount of zinc in the raw water.

[Brief explanation of drawings]

FIG. 1 is a flow sheet that combines the pretreatment method of the present invention and the concentration method using a reverse osmosis membrane device. Second
The figure is a graph showing the relationship between PH and the required amount of caustic soda when PH titration is performed on wastewater from a certain factory using caustic soda. Code explanation, 2... Stirring tank, 5... Sedimentation tank, 7...
...Intermediate storage tank, 9...Super pump, 11...Super tank, 14...Super water tank, 16...Booster pump, 18...Precision filter, 20...High pressure pump, 22...Reverse osmosis module, 27 ...Dehydration equipment.

Claims (1)

[Claims] 1 When concentrating the zinc component from strongly acidic zinc wastewater containing aluminum ions with a pH of 4 or less, first adjust the pH of the wastewater to 5.5 to 6.5 with an alkaline agent, and then separate and remove the generated floc-like solids. , further readjust the pH of the liquid to 3.5 to 5.5,
A pretreatment method for concentrating zinc wastewater to concentrate zinc components.