The invention relates to a method of manufacturing a sorbent suitable for the removal of metal from a hot gas stream, starting from a calcium- and kaolin-comprising base material such as residues released during the produc- tion of paper and during the working up of waste paper for reuse. Within the scope of the invention, a hot gas stream is understood to be a gas stream in which the metals or metal compounds to be removed are present as vapour.

It is known from the prior art that paper or resi- dues being released during the working up of waste paper for reuse, comprise mineral components, such as calcium and kaolin. It is also known to use kaolin or calcium hydroxide, respectively, for the removal of heavy metals from a gas stream. It has therefore indeed been suggested to produce a sorbent starting from a calcium- and kaolin- comprising base material, in particular paper or residues being released during the working up of waste paper for reuse. The advantage would be, that one could economize on the kaolin which is only available in finite amounts, and which is moreover rather costly.

To this end a method is suggested in accordance with the invention, which is characterized in that the base material is thermically treated in a fluidized bed above which there is a freeboard, wherein the fluidized bed is operated at a temperature between 720 and 850°C, and the temperature in the freeboard is maintained at 850°C or below, whereby the base material converts to a substantially metakaolin-comprising substance, forming the sorbent.

Surprisingly it has been shown that in contrast with known mixtures of sorbents, which are individually substantially directed at the removal of some metals, such as kaolin and calcium hydroxide, the sorbent obtained in accordance with the invention not only efficiently removes metals such as, for instance, lead, cadmium, and lead in

the presence of cadmium, but is also effective in removing cadmium in the presence of lead. As already known, the presence of more than one metal in a gas stream to be cleaned, generally has a negative effect on the metal removal efficiency of a prior art sorbent directed at one of those metals. This is not the case with the sorbent according to the invention, with the result that the sor- bent according to the invention is very widely applicable.

It has moreover been shown that the sorbent according to the invention is substantially functional because of its binding via a reactive sorption mechanism rather than removal in accordance with the less desirable pore-diffu- sion mechanism. In contrast with known mixtures of kaolin or calcium hydroxide which act as sorbent, the sorbent according to the invention also appeared to be effective when chlorine-comprising components are present in the gas stream.

The results of the invention can be achieved par- ticularly effectively if the temperature of the freeboard is maintained at 800°C or below. It is especially desir- able that the temperature of the freeboard is kept lower than the temperature of the fluidized bed.

It is to advantage that together with the base material sand grains with a diameter in the region of 0. 5- 2. 0 mm are introduced into the fluidized bed. The sand grains are able to promote heat transfer, allowing quick and correct temperature adjustment in the fluidized bed and in the freeboard. In addition, these sand grains may play a supporting role in obtaining sorbent particles hav- ing a diameter of at least 0. 5 mm due to the fact that the core of said particles are provided with a sand grain.

These relatively heavy particles are formed in the fluidized-bed process as by-product of the actual powdery sorbent, but they are suitable for use as sorbent in very simple gas cleaning applications. Such heavy particles can be introduced quite easily into a gas stream and removed again after use, without the need for complex introduction and removal apparatuses.

The sorbent obtained by the method according to the invention is preferably formed such that at least 80% of the particles have a diameter of at least 2 m. This is a large enough diameter to allow the powdery sorbent, after it has been used for the removal of metal from the gas stream, to be removed from the gas stream by means of standard dust collecting techniques, so that the costs can be kept low.

Preferably the sorbent according to the invention comprises at least approx. 20% by weight of metakaolin. In the opinion of the inventors, this component promotes the reactive sorption quality of the sorbent according to the invention.

Characteristic of the powdery sorbent according to the invention is that indistinguishably present in each metakaolin-comprising particle there are also micron scale calcium-comprising compounds. This was shown by measuring with an electron microscope equipped with X-ray analysis (EDX) by which means calcium, aluminium and silicon can be analyzed. This measurement has shown that in the metakaolin-comprising particle one cannot differentiate between the areas of calcium compounds and metakaolin. It is believed that with regard to activity, this aspect of the sorbent according to the invention distinguishes it from the known sorbents by the simultaneous trapping of different metals from a hot gas stream, either in the pre- sence or not in the presence of anions such as chloride, such that for all these metals a good metal removal effi- ciency is provided.

The calcium-comprising compounds, in particular calcium carbonate and calcium hydroxide have the advantage that when the sorbent is used at high gas temperatures (for example approx. 1000°C), the decomposition of said calcium compounds cause the sorbent particles to disinte- grate, with the result that the sorbent's metal removal efficiency can be increased. It has been shown, moreover, that the sorbent according to the invention possesses cementing properties which are retained even after use while, in addition, the leaching properties of the sorbent

after application are particularly favourable, so that reuse in the building trade would seem possible.

The remarkable properties of the sorbent according to the invention become further apparent when the sorbent is used for the cleaning of a gas stream in general. The sorbent is then especially suitable for the, at least, partial removal of SOx from the gas stream and also for the, at least, partial removal of dioxins from the gas stream. As mentioned above, the sorbent is also effective in the removal of anions.

The invention will now be further elucidated with reference to a few exemplary embodiments in which the sor- bent according to the invention is used and a sorbent in a composition according to the prior art.

EXAMPLE 1 Various heavy metals and combinations of heavy metals comprised in a gas stream, are subjected to a clea- ning process using the following sorbents : kaolin alone, calcium hydroxide alone, kaolin in combination with cal- cium hydroxide, and the sorbent according to the inven- tion. The various sorbents exhibited the following activ- ity.

TABLE A Sorbent Kaolin Ca(OH) 2 Kaolin Sorbent acc. to alone alone and invention Ca (OH), Metal Lead alone good poor good good Cadmium alone poor good good good Lead in pres. of cadmium * * good good Cadmium in pres. of lead poor * moderate good * = not measured It was further shown that, in contrast with the sorbent of the prior art, the sorbent according to the invention exhibits on all metals a so-called reactive

sorption mechanism, which has a favourable effect on the leaching properties. The sorption results shown in the above Table were repeated, wherein the gas also contained chlorine-comprising compounds. Even under those conditions the sorbent according to the invention appeared to be still active.

EXAMPLE 2 The sorbent according to the invention is applied for the removal of anions from a gas stream, such as anions from which SOx can be formed. The gas stream com- prised 4000 mg SOx (calculated as SO2) per Nm3 waste gas.

To this gas stream 90 g/Nm3 sorbent according to the invention is added. After sorption of the SOx, the SOx concentration in the cleaned waste gasses was less than 10 mg per Nm3 waste gas. Thus the removal efficiency is better than 99%.

The SOx-loaded sorbent is shown to be better resistant to leaching out of this compound than other media. In Table B below, the results are shown to compare sulphate leachability when using the sorbent according to the invention and when using portland cement (OPC). To this end sulphur-containing fly ash is added to a SOx- loaded sorbent according to the invention, after which the leachability of sulphate is measured. As comparison a similar amount of sulphur-containing fly ash is added to a commercially available portland cement sample. Then, in a rotating pan, water is added to both samples to obtain moist pellets which are allowed to harden for 7 or 28 days, respectively. In this manner the following sulphate leachability results were obtained.

TABLE B Binding agent OPC 70 % sorbent, 30 % OPC Setting time 7 days 28 days 7 days 28 days Sulphate leachability 2240 1100 670 660 (mg/kg)

EXAMPLE 3 The sorbent according to the invention is used also for trapping dioxins from a gas stream. To this end such a gas stream is contacted with the sorbent according to the invention in a concentration of 90 g/Nm3. The gas is con- tacted with the sorbent for less than 10 seconds, after which the sorbent is separated from the gas stream by means of a baghouse filter. The dioxin concentration being introduced was 1.909 ng/Nm3. Of this, 1. 863 ng/Nm3 (that is to say more than 97%) were trapped by the sorbent, collected in the baghouse filter. 0.037 ng/Nm3 (approximately 2%) were present on sorbent particles found after the dust filter, and 0. 009 ng/Nm3 (that is to say less than 0. 5%) were not trapped by the sorbent and were still present in the gas stream.