JP5850294B2 - Waste disposal method - Google Patents

Description

  The present invention relates to a waste treatment method.

When using wastes such as desalted dust, waste incineration ash, and soot discharged from cement production facilities as cement raw materials, wash them with a cleaning solution such as water in order to remove chlorine from the waste. Has been done.
Since the waste contains a trace amount of selenium in addition to the chlorine content, the cleaning liquid for cleaning the waste contains selenium.
Since selenium is a regulated metal, the cleaning liquid containing selenium cannot be discharged as it is as described above.

If the selenium in the washing liquid is present as tetravalent selenium, it forms a hardly soluble metal salt with iron or the like, so that it can be easily separated and removed by means such as coagulation precipitation.
However, when it exists as hexavalent selenium, such a hardly soluble metal salt is not formed, and since hexavalent selenium is not adsorbed on an ion exchange resin or the like, it is difficult to remove it effectively. It is.

  Therefore, as a method for removing such hexavalent selenium, for example, a reducing agent such as an iron salt is mixed in a cleaning solution containing selenium, and the hexavalent selenium is reduced to tetravalent selenium that can be aggregated and precipitated. There is a method of precipitating and separating as a tetravalent selenium precipitate (Patent Document 1).

  However, in the method using a reducing agent as described above, it is necessary to add a large amount of reducing agent equal to or more than the equivalent in order to sufficiently reduce hexavalent selenium in the liquid, and there is a problem that the processing cost is increased. .

JP2001-9467

  Therefore, in view of the above problems, an object of the present invention is to provide a waste processing method capable of precipitating selenium in a cleaning solution for cleaning waste generated in waste processing at a low cost. .

In order to solve the above problems, a waste processing method according to the present invention includes a cleaning step of cleaning waste by bringing it into contact with a cleaning liquid, and precipitating selenium contained in the cleaning liquid after contacting the waste. A waste treatment method comprising a precipitation step, wherein a reducing agent is added to a washing liquid from which solid components have been removed by solid-liquid separation after contacting the waste in the washing step, and the reducing agent and the A reaction step of reducing the selenium by reacting with selenium contained in a cleaning solution, wherein the precipitation step is a step of precipitating the selenium reduced in the reaction step and an excess reducing agent as a precipitate, At least a part of the precipitate is returned to the washing step so as to be 1% by mass to 5% by mass with respect to the waste.

  In the waste processing method of the present invention, selenium contained in the cleaning liquid generated in the waste cleaning step is reduced with a reducing agent, and the reduced selenium and excess reducing agent are precipitated as precipitates, By returning at least a part of the precipitate to the washing step, which is a step before the addition of the reducing agent, the amount of the reducing agent to be added later can be reduced and the cost of the reducing agent can be reduced. it can.

The exact mechanism by which the amount of reducing agent added later can be reduced by the addition of the precipitate is unknown, but one is to reuse the excess reducing agent contained in the precipitate. This is considered to be because the reducing agent to be added can be suppressed to a small amount.
Furthermore, another reason is that selenium is ionized from the waste by a salt containing selenium contained in the precipitate or an excess reducing agent by adding the precipitate to the waste washing step. This is considered to be because dissolution in the liquid is suppressed.

  Moreover, since at least a part of the precipitate is used in the washing step, the amount of the discharged precipitate is reduced, and the treatment cost of the precipitate can be reduced.

  Moreover, in this invention, it is preferable to make the said reducing agent and the said selenium react on the acidic conditions of pH 5.0 or less in the said reaction process.

  By reacting the reducing agent with selenium in the liquid under acidic conditions, the reducing agent becomes easier to dissolve in the liquid, and the reactivity of the reducing agent increases. Therefore, reduction of hexavalent selenium with a small amount of reducing agent is required. Can do.

Moreover, in this invention, it is preferable that the said reducing agent is at least 1 or more selected from the group which consists of ferrous chloride, ferrous sulfate, and ferrous nitrate.
When the reducing agent is used, the reducing property is particularly good under acidic conditions, and when these reducing agents are used, the reducing agent is easily dissolved under acidic conditions. It is possible to effectively reduce hexavalent selenium with this reducing agent.

  ADVANTAGE OF THE INVENTION According to this invention, the selenium in the washing | cleaning liquid produced in the waste processing method can be precipitated at low cost.

The flowchart which shows the processing method of the waste of this embodiment.

Embodiments according to the present invention will be described below.
The waste processing method of this embodiment will be described based on the flow shown in FIG.

  In the waste processing method of this embodiment, the waste is brought into contact with washing water such as water to wash the waste, and selenium contained in the waste generated in the washing step 1 is dissolved. A reducing agent addition step 2 for adding a reducing agent to the cleaning solution, and the cleaning solution to which the reducing agent has been added in the reducing agent addition step 2 is placed under predetermined reaction conditions and dissolved in the reducing agent and the cleaning solution. A reaction step 3 for reducing selenium, a precipitation step 4 for precipitating the reduced product reduced in the reaction step 3, and a return step 5 for returning the precipitate obtained in the precipitation step 4 to the washing step 1. It has.

The processing method of the present embodiment as described above will be specifically described below.
Examples of the waste treated by the treatment method of the present embodiment include waste such as desalted dust discharged from a cement manufacturing facility such as a cement kiln.

The waste contains water-soluble substances such as chlorine and sulfate, and when the waste is used as a cement raw material, it is necessary to remove these water-soluble substances.
Therefore, in the cleaning step 1, the water-soluble substance is removed from the waste by bringing the waste into contact with cleaning water such as water.

The washing process performed in the washing step 1 is, for example, waste and washing water by stirring the waste and washing water in a washing tank under treatment conditions of 20 to 50 ° C. and 10 minutes to 3 hours. It is preferable to carry out the cleaning by bringing them into contact with each other.
The amount of cleaning liquid to be mixed can be adjusted according to the type of waste to be treated and the amount of water-soluble substances contained. For example, when desalted dust that is waste is washed with water as cleaning water It is preferable to mix about 3 to 10 parts by weight, preferably about 5 parts by weight of water with respect to 1 part by weight of desalted dust.

The washing water mixed with the waste in the washing step may be any liquid that can wash the water-soluble substance. In addition to tap water such as tap water, treated water obtained by various wastewater treatments is used. May be.
In the washing step, water-soluble substances such as chlorine are removed from the waste, but at the same time, selenium contained in the waste is dissolved in the washing liquid.
At this time, a part of selenium is present in the cleaning liquid as hexavalent selenium.

From the cleaning step 1, by bringing the waste and the cleaning liquid into contact with each other, the water-soluble substance and selenium are dissolved in the cleaning liquid, and the cleaning liquid in which the waste is mixed is discharged.
In order to remove the solid component of the waste, the cleaning liquid may be subjected to a solid-liquid separation process in the solid-liquid separation step 6 to remove the solid component.

  In addition, you may utilize the solid component removed by the said solid-liquid separation process 6 as a cement raw material separately. In this case, the selenium concentration contained in the cement is preferably used as a raw material in a range not exceeding a predetermined reference concentration.

  Before the reducing agent is added in the reducing agent adding step 2, the pH adjusting agent is added in the pH adjusting step 8 to the cleaning solution from which the solid components have been removed through the solid-liquid separation step 6, and the cleaning solution is made acidic. Adjust to.

The acidic pH adjusted in the pH adjusting step 8 is pH 5.0 or less, preferably pH 3.0 to 5.0, and more preferably pH 4.0.
When the pH of the cleaning liquid is within the above range, the reducing agent added in the reducing agent addition step 2 is easily dissolved, and a reduction reaction can be effectively caused even with a small amount of reducing agent.

  As said pH adjuster, inorganic acids, such as hydrochloric acid, a sulfuric acid, nitric acid, can be used.

Next, in the reducing agent adding step 2, a reducing agent is added to the cleaning liquid adjusted to be acidic in the pH adjusting step 8.
As the reducing agent, any reducing agent that reduces hexavalent selenium to tetravalent selenium can be appropriately selected and used.
Examples of such a reducing agent include ferrous chloride, ferrous sulfate, and ferrous nitrate.
Among these reducing agents, ferrous chloride is used from the viewpoint that it reacts with components other than tetravalent selenium and does not easily generate precipitates other than selenium-containing precipitates, and also does not conflict with emission standards. It is preferable.

  In general, the reducing agent is added in an excess amount equal to or more than an equivalent amount in order to reliably reduce selenium in the cleaning liquid. As described later, the precipitate obtained in the precipitation step 4 is added to the cleaning step. When returned to step 1 and mixed with the cleaning liquid, a sufficient amount of reducing ability can be obtained as compared with the case where the precipitate is not mixed, so that a small amount is sufficient.

In addition, since the pH of the cleaning liquid is adjusted to be acidic in the pH adjusting step 8, the reducing agent tends to exist as ions (iron ions) in the liquid.
Therefore, the reduction reaction can be performed efficiently in the next reaction step.

In the reaction step 3, the cleaning liquid to which the reducing agent is added reacts the reducing agent with hexavalent selenium in the cleaning liquid to reduce hexavalent selenium to tetravalent selenium.
In the reaction step 3, the reduction reaction is performed at a reaction temperature of 20 ° C. to 50 ° C., preferably 25 ° C. to 40 ° C., under a reaction time of 30 minutes to 300 minutes, preferably 60 minutes to 120 minutes.
Within this range of reaction temperature and reaction time, hexavalent selenium can be sufficiently reduced to tetravalent selenium.

  In the reaction step 3, hexavalent selenium in the cleaning liquid is reduced to tetravalent selenium. The reduced tetravalent selenium is formed as metal selenium fine particles or as a hardly soluble metal salt attached to the surface of the reducing agent. Precipitates in the liquid.

  After adjusting the washing liquid that has undergone the reaction step 3 to an alkaline pH of 7.5 to 10.5 in the pH adjustment step 9, the washing solution is adjusted to 20 to 50 ° C. for 30 minutes in the precipitation step 4. The mixture is allowed to stand for 120 minutes to precipitate the tetravalent selenium precipitated in the liquid as described above, while the supernatant water is recovered as recovered water.

  Sodium hydroxide, potassium hydroxide, lime milk, etc. can be used as a pH adjuster for adjusting the cleaning liquid to be alkaline.

Note that a polymer flocculant may be added to the cleaning liquid after the pH is adjusted to be alkaline in the pH adjusting step.
By adding such a polymer flocculant, a precipitate containing selenium precipitated in the precipitation step 4 can be formed into a larger iron hydroxide floc, and the time required for precipitation can be shortened. There are advantages.

In addition to tetravalent selenium, the precipitated precipitate contains an excessive reducing agent among the reducing properties added excessively in the reducing agent addition step 2.
Part or all of such precipitates are returned to the washing step 1 through the return path in the return step 5.

The return path may be configured to be connected to a cleaning tank or the like in which the cleaning process 1 is performed so that precipitates are mixed in the cleaning liquid in the cleaning process 1 or discarded in the cleaning process 1 You may comprise so that a precipitate may be mixed with the washing | cleaning liquid before mixing with a thing.
In this case, the returned sediment is mixed with the cleaning liquid before being brought into contact with the waste in the cleaning step 1 and returned to the cleaning step 1 as cleaning water.

The amount of the precipitate to be returned can be appropriately adjusted according to the amount of waste treated, but the amount of the precipitate is adjusted to 1% by mass to 5% by mass with respect to the waste in the cleaning process. Return it.
The amount of the precipitate relative to the waste refers to the amount of the precipitate obtained by drying the precipitate under a drying condition of a temperature of 105 ° C. for 3 hours.

  In the treatment method of this embodiment, part or all of the precipitate is returned to the washing step 1 and circulated, so that it is not necessary to collect the precipitate and treat it separately, or the amount to be treated Can be reduced.

  Furthermore, the amount of the reducing agent added in the subsequent reducing agent addition step 2 can be reduced by returning the precipitate to the washing step 1 before the reducing agent addition step 2 as described above.

  Examples of the present invention will be described below.

<< Test 1 >>
Into the mixing tank, 40 kg of desalted dust and 200 kg of water were added and stirred at 30 ° C. for 120 minutes to obtain a slurry. The slurry was subjected to suction filtration using Nutsche.
Hydrochloric acid was added to the liquid component after filtration to adjust to pH 4.0.
To the liquid component whose pH was adjusted, a ferrous chloride solution (a solution in which 35 g of ferrous chloride tetrahydrate was dissolved in 100 g of water) was added so as to be 2.0% by mass with respect to the liquid component, The reduction reaction was carried out in a reaction vessel at 30 ° C. for 3 hours.
Thereafter, sodium hydroxide is added to adjust the pH to 9.5, 10 mg / L of nonionic polymer flocculant (trade name: N100S, manufactured by MT Aqua Polymer Co., Ltd.) is added, and the mixture is allowed to stand for 30 minutes in a precipitation tank. Then, a reduction product generated by the reduction reaction was precipitated.
The supernatant was recovered as treated water, while 1.2 kg-dry was obtained as a precipitate.
The precipitate was dried in a drying furnace at a temperature of 105 ° C. for 3 hours.

<< Test 2 >>
As in Test 1, 40 kg of desalted dust and 200 kg of water were added to the mixing tank, and at the same time, 1 kg-dry of the precipitate obtained in Test 1 was added to the mixing tank and stirred to obtain a slurry. .
Thereafter, the same treatment as in Test 1 was performed, but ferrous chloride was added so as to be 0.5% by mass.

Comparing the selenium concentrations in the collected water from Test 1 (without precipitate addition) and Test 2 (with precipitate added), the recovered water in Test 1 was less than 0.1, and the recovered water in Test 2 was less than 0.1. It was.
That is, while the selenium concentration in the treated water was the same, in Test 2, the amount of reducing agent added was 1/4 that in Test 1.
In each test, since the precipitate was returned to the mixing step, the treatment of the precipitate was unnecessary.

<< Test 3 >>
In the same manner as in Test Examples 1 and 2, 40 kg of desalted dust and 200 kg of water were put into the mixing tank for processing.
In addition, hydrochloric acid was added to the liquid component after filtration, and it adjusted to pH 6.0.
Furthermore, the addition amount of ferrous chloride was added so that it might become 2.0 mass%.
Thereafter, the same treatment as in Test Examples 1 and 2 was performed.
As a result, the selenium concentration in the recovered water of Test Example 3 was 0.45. This was 4 times higher than the selenium concentration in the recovered water obtained as a result of Test Examples 2 and 2.

1 Mixing process 2 Reducing agent addition process 3 Reaction process 4 Precipitation process 5 Return process

Claims (3)

A waste treatment method comprising: a washing step of bringing waste into contact with a washing solution; and a precipitation step of precipitating selenium contained in the washing solution after being brought into contact with the waste,
A reducing agent is added to the cleaning liquid from which solid components have been removed by solid-liquid separation after contacting the waste in the cleaning step, and the reducing agent and selenium contained in the cleaning liquid are reacted to reduce the selenium. With a reaction process,
The precipitation step is a step of precipitating selenium reduced in the reaction step and excess reducing agent as a precipitate;
A waste treatment method, wherein at least a part of the precipitate is returned to the washing step so as to be 1% by mass to 5% by mass with respect to the waste.   The waste treatment method according to claim 1, wherein in the reaction step, the reducing agent and the selenium are reacted under acidic conditions of pH 5.0 or less. The waste treatment method according to claim 1 or 2, wherein the reducing agent is at least one selected from the group consisting of ferrous chloride, ferrous sulfate, and ferrous nitrate.

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