WO2007085559A1

WO2007085559A1 - Method of treating stone sludge

Info

Publication number: WO2007085559A1
Application number: PCT/EP2007/050477
Authority: WO
Inventors: Michael Salzmann; Michel Buser
Original assignee: Ciba Spezialitaetenchemie Holding AG; Ciba Holding AG; Ciba SC Holding AG
Current assignee: BASF Schweiz AG
Priority date: 2006-01-27
Filing date: 2007-01-18
Publication date: 2007-08-02
Anticipated expiration: 2008-07-27

Abstract

The present invention relates to a method of eliminating NO2 and Cr(VI) from stone sludge obtained from washing natural stone and concrete, such as is obtained, for example, in the excavated material from tunnel construction.

Description

Method of treating stone sludge

The present invention relates to a method of eliminating NCV and Cr(VI) from stone sludge obtained from washing natural stone and concrete.

In the construction of tunnels, large amounts of stone material removed from the excavations are produced. Such stone material can often be used further. For that purpose, it is sorted according to size and washed with water in a material separation plant (MSP). The water used is recirculated in the MSP. After the stone material has been washed, the water contains very fine inorganic sludge particles which cannot be recycled. Those sludge particles are separated off and pressed dry. The dewatered sludge needs to be sent for disposal at a special landfill site since it is contaminated by toxic substances.

The method currently used in a material separation plant (MSP) is shown in the form of a flow diagram in Figure 1.

The use of explosives in the blasting operation in the tunnel results in the formation of nitrite (NCV), which can be found in the material that has been blasted out. After blasting, the tunnel wall is lined with sprayed concrete. Residues of concrete consequently get into the material removed from the excavations. The concrete contains hexavalent chromium (Cr(VI)). When the material is washed in the MSP, NO₂ ^" and Cr(VI) pass into the water circulation system and into the sludge particles, which are not suitable for further use. The result of this is that NO₂ ^" and Cr(VI) are also present in the dry-pressed sludge, which needs to be sent for disposal at a landfill site.

A treatment for reduction with FeSO₄ is proposed for the detoxification of Cr(VI)-containing stone sludge, for example, in US 5 304 710.

Limits are set worldwide for the storage of waste at landfill sites. If the pressed sludge is to be stored at an inert material landfill site, for example, in Switzerland, the concentration of NO₂ ^" in the eluate must not exceed the value of 0.1 mg/l. The limit for Cr(VI) in the eluate for storage at an inert material landfill site is 0.01 mg/l. Such limits are often exceeded in the pressed sludge. The sludge therefore cannot be stored at an inert material landfill site but needs to be disposed of at a separate landfill site (bioreactor landfill site). Storage at such a landfill site is associated with considerable effort expenditure and appreciable costs. There is accordingly a need for a method by which the sludge produced can be so treated that, at most, extremely small traces of NO₂ ^" and Cr(VI) are still present after treatment, but are in any event below the legally prescribed limits.

It has now been found that the problem can be solved by inserting, between the sludge separation and the filter press, a reaction unit that is provided with means of ensuring thorough mixing and in which treatment with specific chemicals can be carried out. It is unimportant whether that reaction unit is designed for batchwise or for continuous operation. It is merely necessary to ensure that thorough mixing that is as uniform as possible is achieved, for example by means of a stirring device. Suitable means and measures are known to the person skilled in the art.

The present invention relates to a method of eliminating NO₂ ^" and Cr(VI) in wet sludge obtained from washing natural stone and concrete, which comprises the steps of a) adjusting the pH value to less than pH 4 using an inorganic acid and allowing reaction to take place; b) adding an oxidising agent and allowing the mixture to react; c) adjusting the pH value to from pH 6 to 12; and d) separating the solid phase from the liquid phase.

In the washing of natural stone and concrete, it will be understood that the concentrations of NO₂ ^" and Cr(VI) will not be constant but may vary within a wide range, such variation not, however, adversely affecting the basic procedure.

Typically, the concentrations of NO₂ ^" prior to treatment are in the range from 0.1 to 1000 mg/l and those of Cr(VI) from 0.01 mg/l to 10 mg/l. It is, however, also possible, using the present method, to treat and eliminate concentrations departing from those ranges.

Preferably, the wet sludge originates from the excavated stone material from tunnel construction. For example, the wet sludge has a dry substance content of from 1 to 65 percent by weight, preferably from 10 to 60 and especially from 30 to 50 percent by weight.

In any event, the wet sludge should still be readily pumpable and stirrable. If necessary, additional water may be added in order to ensure improved processability.

Prior to treatment, the wet sludge in many cases has a pH value of from 3 to 14, preferably from 6 to14 and especially from 10 to 13.

The nature of the inorganic acid that is added in step a) is not important per se. The addition of H₂SO₄, HCI, HI, HCIO₄, HBr, HNO₃ or H₂SO₃ has proved advantageous. Special preference is given to the use of H₂SO₄.

The amount of acid added is, in principle, governed by the initial pH of the wet sludge and the desired pH that is to be set. Normally, from 0.05 mol H⁺ to 0.7 mol H⁺ needs to be added per kg of wet sludge.

The acid may be added in one or more batches or continuously over a specific period of time. When the addition is continuous, it can be carried out, for example, for from 1 minute up to 1 hour, with preference being given to a range of from 5 to 15 minutes.

In step a), the pH value is preferably adjusted to from 1 to 3, especially to pH 2.

The reaction time in step a) may vary within wide limits. Typically, it is from 5 minutes to 2 hours, with preference being given to from 5 to 30 minutes.

The temperature in reaction step a) may vary within a wide range, for example from 5 to 95°C; it is preferably about ambient temperature, for example from 10°C to 30°C. During the reaction, heat may be evolved, resulting in an increase in temperature.

The external pressure is not in itself important for the reaction, but the procedure is preferably carried out at the prevailing normal pressure. - A -

In step b), the oxidising agent used is, for example, NH₂SO₃H, H₂O₂, NaOCI, an oxide of chlorine (CI_xOy), KMnO₄, Na₂CO₃x3H₂O, NaBO₂(OH)₂, HOCI, NaO₂, BaO₂, O₃, NO_x or 1O₃H.

The oxidising agent used is preferably NH₂SO₃H or H₂O₂, especially H₂O₂.

The amount of oxidising agent that needs to be used depends largely on the amount of of NO₂ ^" and of other oxidisable substances that may be present in the wet sludge, as well as on the chosen oxidising agent.

The oxidising agent is added, for example, in an amount of from 0.01 mol/kg of dry substance to 0.5 mol/kg of dry substance, preferably from 0.1 to 0.5 mol/kg of dry substance.

The oxidising agent may likewise be added in one or more batches or continuously over a specific period of time. When the addition is continuous, it can be carried out, for example, for from 1 minute up to 1 hour, with preference being given to a range of from 5 to 15 minutes.

The reaction time in step b) may likewise vary within wide limits. Typically, it is from 5 minutes to 2 hours, with preference being given to from 5 minutes to 30 minutes.

The above conditions apply in respect of the other parameters of the reaction, such as pressure and temperature.

In step c), the pH value is adjusted using, for example, NaOH, Ca(OH)₂, CaO, Ba(OH)₂, BaO, NH₃, CaCO₃, Na₂CO₃, MgCO₃ or Na₃PO₄.

Preferably, the pH is adjusted using NaOH, CaO Or Ca(OH)₂.

The addition may in this instance, too, be carried out in one or more batches or continuously over a specific period of time. When the addition is continuous, it can be carried out, for example, for from 1 minute up to 1 hour, with preference being given to a range of from 5 to 15 minutes. The pH value in step c) may be adjusted to a value of from pH 6 to 12. Preferably, however, it is adjusted to a value of from 6 to 8, especially from 6 to 7.

The separation of the solid phase from the liquid phase can be carried out using any known means.

Examples are filters, presses, suction filters or pressure suction filters. Such means are known to the person skilled in the art and are widely used in material separation.

Typically, all of the process steps a) to d) are carried out at a temperature of from 10°C to 50°C and under normal pressure conditions.

The following Examples illustrate the invention.

Example 1

Wet sludge from a Swiss tunnel-construction project is used. The wet sludge is pumped into a container having a stirring device: wet sludge 375 kg dry substance 40% pH 12.2

Initial amount: NCV concentration in the eluate 0.67 mg/l Initial amount: Cr(VI) concentration in the eluate 0.02 mg/l

Step a: 1 1.5 kg of H₂SO₄ (96%) are added to the reaction mass over a period of 10 minutes with continuous stirring. pH after addition: 2.

Step b:

2.25 kg of NH₂SO₃H are added to the reaction mass over a period of 10 minutes with continuous stirring. The pH drops to 1.43.

Step c:

12.6 kg of NaOH (32%) are added to the reaction mass over a period of 10 minutes with continuous stirring. The pH rises to 6.3 The following concentrations are determined:

NO₂ ^" concentration in the eluate 0.04 mg/l

Cr(VI) concentration in the eluate < 0.001 mg/l

Step d:

The wet sludge specimen is dewatered by means of filtration.

Examples 2 to 4

The procedure as described in Example 1 is followed. The amounts and results are given in Table 1. Table 1

Claims

What is claimed is:

1. A method of eliminating NCV and Cr(VI) in wet sludge obtained from washing natural stone and concrete, which comprises the steps of: a) adjusting the pH value to less than pH 4 using an inorganic acid and allowing reaction to take place; b) adding an oxidising agent and allowing the mixture to react; c) adjusting the pH value to from pH 6 to 12; and d) separating the solid phase from the liquid phase.

2. A method according to claim 1 , wherein the wet sludge originates from the excavated stone material from tunnel construction.

3. A method according to claim 1 , wherein the wet sludge has a dry substance content of from 1 to 65 percent by weight.

4. A method according to claim 1 , wherein, prior to the beginning of the treatment, the wet sludge has a pH value of from 3 to 14.

5. A method according to claim 1 , wherein the inorganic acid used in step a) is H₂SO₄, HCI, HI, HCIO₄, HBr, HNO₃ or H₂SO₃.

6. A method according to claim 1 , wherein, in step a), the pH value is adjusted to from pH 1 to 3.

7. A method according to claim 1 , wherein the oxidising agent used in step b) is NH₂SO₃H, H₂O₂, NaOCI, an oxide of chlorine (CI_xOy), KMnO₄, Na₂CO₃x3H₂O, NaBO₂(OH)₂, HOCI, NaO₂, BaO₂, O₃, NO_x or 1O₃H.

8. A method according to claim 7, wherein the oxidising agent is added in an amount of from 0.01 mol/kg of dry substance to 0.5 mol/kg of dry substance.

9. A method according to claim 1 , wherein, in step c), the pH value is adjusted using NaOH, Ca(OH)₂, CaO, Ba(OH)₂, BaO, NH₃, CaCO₃, Na₂CO₃, MgCO₃ or Na₃PO₄.

10. A method according to claim 1 , wherein, in step c), the pH value is adjusted to from 6 to 7.