Articles:  Alicante, 2004: Frankfurt, 2005: Karlsruhe/Heidelberg, 2006: Brussels, 2007: The EUCYS:

Investigating the removal of heavy metals from processed bottom ash

By Richard Broadbent

European School Bergen
January, 2004

CONTENTS

Abstract                                                 
Background information                          
Introduction                                            
Experiments and results
Developing the froth floatation apparatus 
Bottom ash experiments                         
Processed bottom ash experiments        
Leaching experiments
Discussion and Conclusions
References

ABSTRACT

In Holland household and industrial waste is incinerated to provide electricity. During the process various materials are removed leaving bottom ash – about 25% by mass of the original waste. This ash contains metals such as copper, cadmium and mercury making it an environmental problem.
Initial experiments were performed to find the best approach to removing these metals. Froth flotation and leaching were finally chosen. A series of experiments at different pH values and with different leaching agents was carried out. Froth flotation at pH 4 separated molybdenum and zinc, whilst at pH10 copper was separated. Leaching experiments at pH 4, 7 and particularly at pH10 removed copper. Microbial leaching was not particularly successful. Analysis of the samples was mainly carried out using a scanning electron microscope.

BACKGROUND INFORMATION

The continued growth in the quantity of household and low-grade industrial waste in The Netherlands has lead the government to look at alternatives to ground fill as a method of disposing of this material. Their solution was to set up a series of incinerators to burn the material and to supply energy in the form of electricity. In the Netherlands 11 incinerators burn household and industrial waste and the electricity produced is enough to power over 1 million households all over the Netherlands.

The nearby plant in Alkmaar recycles the annual 450 000 tonnes of waste material generated in North Holland – sufficient to completely fill The Arena stadium in Amsterdam. It produces 37MW of electricity for the national grid, or the equivalent electricity used annually by 75 000 homes.
A large part of the plant exists for to ensure the waste is environmentally friendly, producing materials called fly ash, salts and filter cake. Other materials, which are recovered in the process, include ferrous and non-ferrous metals, which are sold on.

The material that cannot be combusted is known as bottom ash and accounts for about 25% of the waste material burnt by the plant. This bottom ash goes through a second process to remove as much copper and aluminium as possible and the remaining, or processed bottom ash, is then stored at the plant.

As well as containing non-combustible material, and material that hasn’t combusted for a variety of reasons, the bottom ash also contains traces of heavy metals, e.g. copper, molybdenum, cadmium, zinc and mercury and so its use as a basic building material is limited. One of the ash’s uses is as a building material for bridge approach road foundations. Because of the metal content it is a grade 2 building material, meaning that it has to be encased in a liner which stops the metals leaching out, and it has to be used above the water table. The processed bottom ash from Alkmaar is used for this purpose but such uses have environmental implications.

Research work in laboratories in Germany has shown that even under these conditions there is a fairly rapid leaching out of the metals. On site investigation shows a much slower trend. If the metals could be removed from the processed bottom ash before use this situation could be avoided.

INTRODUCTION

The aim of the project was to see if I could separate heavy metals from the processed bottom ash. From my initial research I came up with several possible methods of reducing or removing these metals: dissolving the bottom ash in acid or alkali, using plants to remove the metals (phytoremediation), froth flotation and later on, leaching and microbial leaching.

DEVELOPING THE FROTH FLOTATION APPARATUS

Froth flotation is a difficult procedure that relies upon precise conditions for success. In particular, the gas bubbles must be small and the pH is critical on this scale. I carried out a series of experiments using small and large-scale apparatus to find the best conditions and the best apparatus for this procedure. The first stage involved using an aerator from a fish tank to produce air bubbles and allowing the bubbles to travel through various containers holding the bottom ash. This was not successful as insufficient pressure was generated. However, the detergent used as the frothing agent was found to be suitable. The next stage involved the use of nitrogen as the source of gas bubbles as this was from a pressurised container. Only nitrogen and oxygen were available, not air. This wasn’t too much of a problem as nitrogen, unlike oxygen, is relatively unreactive. Nitrogen proved to be an adequate source of bubbles, but the basic test apparatus was not suitable. I moved on to testing a column using detergent as the frothing agent and nitrogen for the bubbles. The results of these experiments produced the apparatus shown on P. 1 of the appendix. The whole apparatus is about 2m long with a diameter of about 8cm. A later modification is apparatus B also on P. 1. This modification was necessary as the bottom ash was originally placed in the U-tube but became stuck in the drain so that froth flotation was difficult to achieve. Once the change was made the bottom ash was placed in the sinter funnel so that the bubbles rose through the ash and froth flotation could occur more readily.

BOTTOM ASH EXPERIMENTS

DISSOLVING
These early ex`periments were carried out on the bottom ash whilst I was perfecting the froth flotation apparatus. I tried both acid and alkali on unprocessed ash and had the results analysed. They showed little of interest except large peaks for sulphur.

PROCESSED BOTTOM ASH EXPERIMENTS
First of all the processed bottom ash was dried and then crushed to a powder in a pestle and mortar. Several froth flotation experiments were performed using the modified form of the flotation column. Detergent was used as the frothing agent and nitrogen gas was passed through the apparatus as I did not have access to air. The initial trials showed good separation of the material so more experiments were set up at pH7, pH10 and at pH4. The material in the froth and the remaining material, or residue, were analysed using X-ray diffraction and a scanning electron microscope (SEM). Later samples were analysed only with the microscope. I also tried to use oil as a frothing agent, but this was not successful so I returned to the detergent.

RESULTS
The pH 7 experiments showed no separation of heavy metals from the bottom ash. The pH 4 experiments however were more successful. The froth sample showed 2 peaks on the scanning electron microscope corresponding to Molybdenum and Zinc, whereas the residue sample showed no peaks for these two metals. (See P. 2 of the appendix). Also it can be seen that at pH 10 copper was also obtained.

LEACHING EXPERIMENTS

1. LEACHING EXPERIMENTS
a. Leaching experiments based on the method used to extract copper from low-grade ores were set up. About 1kg of processed bottom ash was placed into 3 inverted 1.5 litre drinks bottles which had gauze over the mouth of the bottle. Leaching solutions of pH 4, 7 and 10 were added periodically and the run off collected in the beaker. The run off was dried and sent for analysis.
b. The leaching buffer solutions had been made using ammonium chloride, ammonia solution, and ethanoic acid and sodium ethanoate. The results at pH10 indicated that the ammonia could be the important factor as copper forms a complex with ammonia. I made some more solutions as well as using just ammonia solution and carried out some more leaching experiments as described above.
c. The run off from these results was used to carry out further tests. I added zinc to some of the run off and iron to another sample in an attempt to precipitate out the copper. The resulting solids were dried and sent for analysis. I also tried electrolysis on the run off to see if the copper could be removed this way. Although the graphite rod appeared to show little coloration caused by deposited copper, analysis of the sample did show copper to be present.
d. As research in Germany had shown leaching to occur naturally in a short period of time I set up an experiment outdoors. The bottom ash was placed in a container with small holes in the bottom and the container placed outside above a receptacle used to collect the run off. This allowed the elements to be in direct contact with the bottom ash but no additional chemicals were used.

RESULTS
a. At pH 10 the run off was a deep blue colour. The others were somewhat murky in appearance. Analysis of the run off at pH 10 showed the presence of copper indicating its removal from the bottom ash. See P. 4 of the appendix.
b. Interestingly, the solution producing the most intense blue colour was the buffer made with ammonia and ammonium chloride and not the ammonia solution.
c. The addition of both zinc and iron produced red brown precipitates, more so with the iron (although this could have been caused by oxidation of the iron). Analysis of the results did show copper to be present in both samples. See P. 5, 6 and 7 of the appendix.
d. This is an ongoing experiment and so no results are available at the present time.

2. MICROBIAL LEACHING
a. The extraction of copper uses microorganisms such as Thiobacillus ferrooxidans. An intensive search was made to obtain a specimen sample of these and similar organisms. Suppliers in the UK and The Netherlands were contacted until eventually a supplier in Germany was located. The Thiobacillus was ordered and upon arrival it was cultured in a liquid medium. A leaching experiment was then set up using the processed bottom ash. The Thiobacillus suspension was sprinkled over the bottom ash and the run off collected.
b. Searches on the Internet revealed that bakers yeast was used to extract some metals from ores so I decided to make a yeast mixture and try leaching the bottom ash with this micro-organism.

RESULTS
The culturing of the microorganisms was not too successful and leaching experiments with the Thiobacillus did not appear to work. Analysis of the run off showed no heavy metals present.
Similarly, the use of yeast produced nothing of interest.

DISCUSSION AND CONCLUSION

The experiments on dissolving the samples in acid and alkali produced little in the way of useful results. Both substances were able to dissolve the material. This may be a useful starting point for the selective precipitation of the metal ions, but it was not the main purpose of my experiments. It did, however, give a result that was crucial to the later stages of my research – the lack of sulphur in the residue from the base sample.
The early froth flotation experiments showed the difficulty of carrying out this technique. Once I worked with the long column with the sinter funnel in the new position and using nitrogen from a gas cylinder in place of an aerating machine, I had much more success. The samples were placed on the sinter funnel, thus avoiding the problem of it falling into the drainage point, but having the advantage that the gas bubbles came directly into contact with the sample. This method produced much better separation.
Processed bottom ash was relatively easy to work with; it was relatively homogenous in its original state and, once dried, it was easy to crush to a fine powder. The results from the experiment at pH 4 showed that both zinc and molybdenum had been removed. There is no doubt about the zinc, but there is a possibility that the molybdenum peak shown on the electron scanning microscope graph could actually be sulphur. The reason is that both elements produce peaks at about the same energy in this analytical process.
The expert analysts from the research centre indicate that the shape of the curve is that of molybdenum and not sulphur. In addition, the incineration process should have removed any sulphur and sulphur compounds as waste gases due to pyrolysis and oxidation. There is at least 11% of oxygen in the incinerator to carry out the process of oxidation and temperatures in the incinerator reach around 900єC. This absence of sulphur is backed up by earlier experiments on dissolving. The scan of the material dissolved in sulphuric acid shows a large sulphur peak – the residue of acid or sulphates formed by chemical reaction. The X-ray diffraction graph similarly shows a large deposit of sulphate in the acid material. The scan showing the base dissolved material does not show a peak at sulphur. In addition, the SEM results from an earlier froth flotation experiment show little or no sulphur contamination from the detergent. From this I can say that sulphur is not present to any detectable extent in the bottom ash and thus the peak on the froth scan is almost certainly that of molybdenum.
The presence of molybdenum in processed bottom ash is a serious problem for the Dutch authorities – its source is not 100% clear, although it is used in lubricants. Its removal from bottom ash would be of great significance and it appears that froth flotation at pH 4 achieves this.
The leaching of copper and other metals from low-grade ores is well established and I think that this may be a technique applicable to the problem of removing metals from bottom ash. As far as I am aware it has never been tried. The results clearly show that residual copper could be removed by leaching at various pH values as well as by froth flotation. The leaching experiments using ammonia and ammonium chloride were most successful, as was the removal of the copper by the addition of zinc or iron. On a large-scale scrap iron could be used to precipitate out the copper. A rough estimate, using a figure of 1% copper present in bottom ash in The Netherlands, indicates that about 12 000 tonnes of copper could be recovered using this procedure.
The lack of success with the use of microorganisms is frustrating as it could prove to be the most useful technique. Research has been carried out in this area and there is literature that shows there has been success with this technique. In my experiments it would appear that I was unable to culture the micro-organisms to produce a large enough concentration to be effective. If I could develop this skill, then I think that there would be some positive outcomes.
Although the booklets produced by the Huisvalcentrale show that Mercury and Cadmium are present in bottom ash (5% and 2% respectively) I have not detected any in the samples used – either before or after treatment. This may be due to the fact that the samples obtained from the Huisvalcentrale are not sufficiently homogenous and so none was contained in my samples. This is disappointing, as the leaching technique should be suited to removing these metals as they also form complexes in a similar way to copper.
Does this project have any real use? Certainly the removal of any of the metals has a beneficial environmental consequence. Also, if large enough amounts of copper could be removed there may be a commercial advantage. If a leaching agent were found that would remove all of the metals, then the bottom ash would have more commercial uses. It seems that microbial leaching would be the way forward here. Froth flotation is not without its benefits (it is a well-known technique) and should not be dismissed lightly. Again, further research could produce the ideal combination of pH and frothing agent to remove the metals.
Finally, one possible use of the leaching technique could be in the cleansing of the ground from old disused industrial sites where heavy metals have accumulated. It is extremely difficult to render a site safe for building or recreation if it is contaminated with metals.
There is still much to do in this research project but I think that I have made some progress with my original aims.

REFERENCES

Huisvuilcentrale Noord-Holland. Booklets giving basic information about the recycling and incineration processes.
The management of MSWI residues in the Netherlands. Stefan Rutten, Lisbon 10/12/01

Leaching of copper ore with microorganisms. www.personal.psu.edu/faculty

Mining with bacteria. Specimen papers, OCR 2000

Cadmium. Jozef Plachy, 1997

Mercury and Cadmium fact sheet. US department of the interior. 12/09/01

www. Mcgill.co./publications

http://alfa.ist.utl.pt/~cpqutl/Resear3.html

Exploiting the genetic and biochemical capacities of bacteria for the remediation of heavy metal pollution. Marc Valls and Victor de Lorenzo, August 2002

Microbial leaching of metals by Helmut Brandl. Zurich.