This invention relates to a method for preparing a sorbent, in particular a method for preparing sorbents comprising copper sulphide. Copper sulphide containing sorbents are useful in removing heavy metals from fluid streams. Heavy metals such as mercury are found in small quantities in fluid streams such as hydrocarbon or other gas and liquid streams. Arsenic and antimony may also be found in small quantities in hydrocarbon streams. Mercury, in addition to its toxicity, can cause failure of aluminium heat exchangers and other processing equipment. Therefore there is a need to efficiently remove these metals from fluid streams, preferably as early as possible in the process flowsheet.

Sorbents comprising CuS are known to be effective for capturing mercury by the following reaction:

2 CuS + Hg → HgS + Cu ₂ S

Copper sulphide is conventionally formed in the sorbents either in situ by reaction with hydrogen sulphide (H2S) present in the fluid stream, or by pre-sulphiding again with hydrogen sulphide.

WO2009/101429 discloses a method for making the absorbent comprising the steps of: (i) forming a composition comprising a particulate copper compound capable of forming copper sulphide, a particulate support material, and one or more binders, (ii) shaping the composition to form an absorbent precursor, (iii) drying the absorbent precursor material, and (iv) sulphiding the precursor to form the absorbent. The sulphiding agent used to sulphide the absorbent precursor may be one or more sulphur compounds such as hydrogen sulphide, carbonyl sulphide, mercaptans and polysulphides, or mixtures of these. Hydrogen sulphide is preferred. However, the sulphiding method using these sulphiding agents, if not carefully controlled, can lead to in-homogeneous product and impaired physical properties. Moreover hydrogen sulphide is a toxic gas and complex control measures are necessary for sulphiding at large scale. Therefore there is a need to seek alternative methods for such products that are inherently safer, simpler and offer improved product homogeneity.

US5245106 discloses a method of eliminating mercury or arsenic from a fluid in the presence of a solid mass for the recovery of mercury and/or arsenic. The mass contains copper and sulphur at least partly in the form of copper sulphide and results (a) from the incorporation of a copper compound other than a sulphide into a solid mineral support, (b) calcination of the product obtained in stage (a), and bringing the product obtained previously into contact with elementary sulphur and (d) a heat treatment.

We have found that particulate elemental sulphur is surprisingly effective in sulphiding particulate copper compounds when heated under an inert gas.

Accordingly the invention provides a method for preparing a sorbent comprising the steps of:

(i) Mixing together a particulate copper compound capable of forming copper sulphide, a particulate support material, particulate elemental sulphur and one or more binders,

(ii) shaping the mixture,

(iii) drying the shaped mixture to form a dried sorbent precursor, and

(i) heating the precursor to a temperature in the range 100-440°C under an inert gas to react the elemental sulphur with the copper compound, thereby forming copper sulphide and form the sorbent.

The invention further provides a copper sulphide-containing sorbent obtainable by the method and the use of the sorbent in removing heavy metals from heavy metal-containing fluid streams. By "sorbent" we include absorbent and adsorbent.

By "heavy metal" we include mercury, arsenic, selenium, cadmium and antimony.

The particulate copper compound suitable for use in the sorbent is one that may be readily sulphided such as copper oxide and/or basic copper carbonate. One or more sulphidable copper compounds may be present. The particulate copper compound may be selected from basic copper carbonate, copper hydroxide, copper oxide or mixtures thereof. A preferred particulate copper compound comprises basic copper carbonate as it appears to be more reactive with elemental sulphur than copper oxide. The particulate copper compound may be commercially sourced or may be generated, e.g. by precipitation from a solution of metal salts using alkaline precipitants. Thus the particulate copper compound may be made by precipitating copper-hydroxycarbonate and optionally zinc-hydroxycarbonate using an alkali metal carbonate and alkali metal hydroxide precipitant mixture, followed by washing and drying the precipitate. Thus the particulate copper compound may include one or more of azurite Cu ₃ (C0 ₃ )2(OH) ₂ ; malachite Cu ₂ C0 ₃ (OH)2; zincian malachite Cu ₂ -xZn _x C0 ₃ (OH)2; rosasite Cu ₂ _ _x Zn _x C0 ₃ (OH) ₂ , aurichalcite Cu ₅ . _x Zn _x (C0 ₃ ) ₂ (OH) ₆ and hydrotalcite-type materials where alumina is included during the precipitation, e.g. Accordingly, the particulate copper compound may be one or more compounds selected from copper oxide, basic copper carbonate, and precipitated materials comprising copper basic carbonate and zinc basic carbonate. The particulate copper compound is desirably in the form of a powder, more a preferably a powder with an average particle size, i.e. D ₅₀ , in the range 5-100μηι, especially 10-50μηι. The dried sorbent precursor preferably comprises 10-70% by weight, preferably 10-50% by weight, of the particulate copper compound.

Unlike US5245106, there is no calcination of the copper compound support material and one or more binders before contacting these with the particulate elemental sulphur.

Particulate elemental sulphur is mixed with the particulate copper compound. The elemental sulphur is desirably in the form of a powder, more preferably a powder with a D[v, 0.5] particle size in the range 5-1 ΟΟμηι, especially 10-50μηι. Using powders with similar or smaller average particle size than that of the particulate copper compound may be effective in increasing their conversion to copper sulphide. The copper : elemental sulphur (Cu:S) atomic ratio in the dried sorbent precursor may be in the range 1 :0.5 to 1 :3.0, but is preferably 1 :1 to 1 :2, more preferably 1 :1 .3 to 1 :1 .7. Ratios in the precursor above 1 :1 provide better levels of sulphidation of the particulate copper compound but amounts above 1 :2 leave un-reacted sulphur in the sorbent which may be released during use, which may be undesirable.

The support may be any inert support material suitable for use in preparing sorbents. Such support materials include alumina, metal-aluminate, silica, titania, zirconia, zinc oxide, aluminosilicates, zeolites, metal carbonate, carbon, or a mixture thereof. The support material offers a means to adapt the physical properties of the sorbent to the duty. Thus the surface area, porosity and crush strength of the sorbent may suitably be tailored to its use.

Furthermore, the presence of support particles can increase the strength and durability of the sorbent composition by acting as a diluent. The sorbent composition is then better able to retain its physical integrity during the sulphiding process, which causes a volumetric change in the copper compound as the copper sulphide is formed. Support materials are desirably oxide materials such as aluminas, titanias, zirconias, silicas and aluminosilicates, or mixtures of two or more of these. Hydrated oxides may also be used, for example alumina trihydrate or boehmite. Particularly suitable supports are aluminas and hydrated aluminas, especially alumina trihydrate. The support is desirably in the form of a powder, more preferably a powder with a D[v, 0.5] particle size in the range 1 -100μηι, especially 5-20μηι.

Binders that may be used to prepare the shaped sorbent include clays such as bentonite, sepiolite, minugel and attapulgite clays; cements, particularly calcium aluminate cements such as ciment fondu; and organic polymer binders such as cellulose binders, or a mixture thereof. Particularly strong shaped units may be formed where the binder is a combination of a cement binder and a clay binder. In such materials, the relative weights of the cement and clay binders may be in the range 1 :1 to 3:1 by weight (first to second binder). The total amount of the binder in the dried sorbent precursor may be in the range 5-30% by weight. The one or more binders are desirably in the form of powders, more preferably powder with with a D[v, 0.5] particle size in the range 1 -100μηι, especially 1 -20μηι.

Other components may also be present in the sorbent to enhance the physical properties of the sorbent. Other such additives include zinc compounds such as zinc oxide, zinc carbonate or zinc hydroxycarbonate, or other transition metal compounds, which may become sulphided during manufacture. However, where high water-tolerance of the sorbent is required, the total metal sulphide content of the sorbent, other than copper sulphide, is preferably < 5% wt, more preferably < 1 % wt. In particular, the zinc sulphide content of the sorbent is preferably < 5% by weight, more preferably <1 % wt, most preferably < 0.5% wt, especially < 0.1 % wt (based on the sulphided composition).

In a preferred embodiment, the sorbent consists essentially of copper sulphide, a support material and one or more binders. In a particularly preferred embodiment, the sorbent comprises 5-50% by weight of one or more particulate sulphided copper compounds, 30-90% by weight of a particulate support material, and the remainder one or more binders, wherein the metal sulphide content of the sorbent, other than copper sulphide, is < 5% by weight. A particularly preferred sorbent composition comprises 20-40% by weight in total of one or more particulate sulphided copper compounds, a particulate hydrated alumina support material, bound together with a cement binder and a clay binder, wherein the zinc content of the sorbent is < 5% by weight.

The mixture of particulate copper compound, particulate elemental sulphur, particulate support and one or more binders is shaped and dried to form a sorbent precursor. Shaping may be by pelleting, extruding or granulating. Hence, sorbent precursor pellets may be formed by moulding a powder composition, generally containing a material such as graphite or magnesium stearate as a moulding aid, in suitably sized moulds, e.g. as in conventional tableting operation. Alternatively, the sorbent precursor extrudates may be formed by forcing a suitable composition and often a little water and/or a moulding aid as indicated above, through a die followed by cutting the material emerging from the die into short lengths. For example extrudates may be made using a pellet mill of the type used for pelleting animal feedstuffs, wherein the mixture to be pelleted is charged to a rotating perforate cylinder through the perforations of which the mixture is forced by a bar or roller within the cylinder: the resulting extruded mixture is cut from the surface of the rotating cylinder by a doctor knife positioned to give extruded pellets of the desired length. Alternatively, sorbent precursor granules, in the form of agglomerates, may be formed by mixing a powder composition with a liquid, such as water, insufficient to form a slurry, and then causing the composition to agglomerate into roughly spherical granules in a granulator. The pellets, extrudates or granules preferably have a length and width in the range 1 to 25 mm, with an aspect ratio (longest dimension divided by shortest dimension) < 4.

The different shaping methods have an effect on the surface area, porosity and pore structure within the shaped articles and in turn this often has a significant effect on the sorption characteristics and on the bulk density. Thus beds of sorbents in the form of moulded pellets may exhibit a relatively broad absorption front, whereas beds of granulated agglomerates can have a much sharper absorption front: this enables a closer approach to be made to the theoretical absorption capacity. On the other hand, agglomerates generally have lower bulk densities than tableted compositions. Furthermore, in view of the presence of elemental sulphur, methods involving small amounts of water are preferred to avoid possible sulphate formation, which is undesirable. Accordingly, it is preferred to make the shaped units in the form of agglomerates and thus a preferred shaping method involves granulating the mixture of particulate copper compound, particulate elemental sulphur, particulate support and binder in a granulator. Granules with a diameter in the range 1 -15 mm are preferred.

The shaped precursor is dried before sulphiding. Drying temperatures up to 120°C may be used. If desired, the drying step may be either separate from, or contiguous with, the heating step that causes the particulate elemental sulphur to react with the particulate copper compound.

The heating step should be performed under an inert gas so that undesirable side reactions are minimised. By "inert" we mean that the gas is generally inert towards the copper and sulphur. Thus the inert gas is preferably essentially free of oxygen or reductants such as hydrogen or carbon monoxide. Furthermore, for safety reasons, preferably the gas should not support combustion. Thus suitable inert gases may be selected from nitrogen, argon and carbon dioxide. Nitrogen is preferred.

The shaped mixture is heated to a maximum temperature in the range 100-440°C. Lower temperatures require lengthy sulphidation times, while high temperatures risk excessive loss of elemental sulphur by evaporation or sublimation. Therefore preferably the shaped mixture is heated to a maximum temperature in the range 150 to 300°C, more preferably 150°C to 250°C, most preferably 180 to 250°C, and especially 200 to 250°C. The dwell time at this maximum temperature may be in the range 0.5 and 24 hours, but it preferably between 0.5 and 8 hours. The heating and cooling rates may be in the range 1 -10°C/minute. Preferably > 50% wt of the copper present in the sorbent is sulphided, more preferably

> 75% wt, more preferably > 95% wt. Essentially all of the sulphided copper in the sorbent is desirably in the form of copper (II) sulphide, CuS. The sorbent may be used to treat both liquid and gaseous fluid streams containing heavy metals, in particular fluids containing mercury and/or arsenic. In one embodiment, the fluid stream is a hydrocarbon stream. The hydrocarbon stream may be a refinery hydrocarbon stream such as naphtha (e.g. containing hydrocarbons having 5 or more carbon atoms and a final atmospheric pressure boiling point of up to 204°C), middle distillate or atmospheric gas oil (e.g. having an atmospheric pressure boiling point range of 177°C to 343°C), vacuum gas oil (e.g. atmospheric pressure boiling point range 343°C to 566°C), or residuum (atmospheric pressure boiling point above 566°C), or a hydrocarbon stream produced from such a feedstock by e.g. catalytic reforming. Refinery hydrocarbon steams also include carrier streams such as "cycle oil" as used in FCC processes and hydrocarbons used in solvent extraction. The hydrocarbon stream may also be a crude oil stream, particularly when the crude oil is relatively light, or a synthetic crude stream as produced from tar oil or coal extraction for example.

Gaseous hydrocarbons may be treated using the process, e.g. natural gas or refined paraffins or olefins, for example. Off-shore crude oil and off-shore natural gas streams in particular may be treated with the sorbent. Contaminated fuels such as petrol or diesel may also be treated. Alternatively, the hydrocarbon may be a condensate such as natural gas liquid (NGL) or liquefied petroleum gas (LPG), or gases such as a coal bed methane, landfill gas or biogas. Gaseous hydrocarbons, such as natural gas and associated gas are preferred.

Non-hydrocarbon fluid streams which may be treated include nitrogen, carbon dioxide, which may be used in enhanced oil recovery processes or in carbon capture and storage, solvents for decaffeination of coffee, flavour and fragrance extraction, solvent extraction of coal etc. Fluids, such as alcohols (including glycols) and ethers used in wash processes or drying processes (e.g. triethylene glycol, monoethylene glycol, Rectisol™, Purisol™ and methanol), may be treated by the inventive process. Mercury may also be removed from amine streams used in acid gas removal units. Natural oils and fats such as vegetable and fish oils may be treated, optionally after further processing such as hydrogenation or transesterification e.g. to form biodiesel.

Other fluid streams that may be treated include the regeneration gases from dehydration units, such as molecular sieve off-gases, or gases from the regeneration of glycol driers.

Preferably the absorption of heavy metal is conducted at a temperature < 150°C, preferably < 120°C in that at such temperatures the overall capacity for heavy metal absorption is increased. Temperatures as low as 4°C may be used. A preferred temperature range is 10 to 80°C. The gas hourly space velocity through the sorbent may be in the range normally employed.

In use, the sorbent may be placed in an absorption vessel and the fluid stream containing heavy metal is passed through it. Desirably, the sorbent is placed in the vessel as one or more fixed beds according to known methods. More than one bed may be employed and the beds may be the same or different in composition.

The invention is further described by reference to the following Examples.

Example 1 . Preparation of sorbent

Raw materials:

35 parts by weight Basic copper carbonate powder. D50 12-20μηι.

51 parts by weight Aluminium trihydrate (ATH) powder (<95μηι).

14 parts by weight Ciment Fondu cement powder (D[v, 0.5] 27.4μηι).

14 parts by weight Attagel 50 clay powder (<25μηι).

Precipitated sulfur powder, 99.5 % (D[v, 0.5] 25.5μηι).

The basic copper carbonate, ATH, cement and clay were combined with elemental sulphur. Granules were prepared with different atomic ratios of copper to sulphur (1 :0.7, 1 :1 , 1 :1 .3 and 1 :1 .5) using an Eirich™ R02 granulator. The granules were dried at 105°C for 10 minutes then sieved to provide a granule size of 2.80 - 4.75 mm.

The sorbent precursors were charged to either a 4 mm Internal Diameter (ID) reactor tube (stainless steel, Sulfinert coated) or a 20 mm ID reactor tube (stainless steel, Sulfinert coated) with a plug of glass wool placed at the bottom of the sample. 100% vol nitrogen was used as the treatment gas. If required, a mass spectrometer was connected to the exit of the reactor tube to monitor the process gas. The heat treatment consisted of a temperature programmed ramp from ambient temperature up to the selected maximum temperature at a rate of

5°C/minute, followed by a hold at this temperature for 1 .5 hours, unless otherwise stated.

The sorbents were analysed before/after use as follows; a) sulphur and carbon contents

Sorbents were ground and sieved to a particle size of less than 250 μηι and analysed using a combustion method in which the samples were combusted in pure oxygen at 1350°C to convert carbon and sulphur into their respective oxides, which were quantified by infra-red- spectroscopy using LECO apparatus. b) S8 Leaching Determination

Levels of any unreacted elemental sulphur that remained in the sorbents were determined by a leaching test. In this procedure, 5 g of sorbent was refluxed in n-hexane for four hours. Then, the sample was cooled to room temperature and the n-hexane was decanted. The solid sample was then dried overnight before being analysed by LECO for its sulphur content. The leached sample is compared with the sulphur content before the leaching test to give a measure of unreacted sulphur present.

c) crystalline composition The sorbents were ground and sieved to a particle size of less than 250 μηι and analysed by X- ray diffraction (XRD) to obtain phase information. A Siemens D5000 Diffractometer was used with a scan range of from 10° to 130° with a step size of 0.044°. Alternatively a Bruker AXS D8 Diffractometer was used with a scan range of from 10° to 130° with a step size of 0.02°. In addition to phase information, Rietveld analysis was carried out to estimate the weight percentage and crystallite size of each phase detected.

Granules of sorbent precursors with Cu:S atomic ratios of 1 :0.7, 1 :1 , & 1 :1 .3 & 1 :1 .5 were heated to either 180°C, 190°C or 215°C for 1 .5 hours. Sulphur and carbon contents were determined. The results are given in Table 1 . The results show that both sulphur and carbon content of the sorbents went down as the treatment temperature was increased. The decrease in carbon content indicated an increase in conversion of basic copper carbonate (via loss of C0 ₂ ). The loss in sulphur may be attributed to formation of S0 ₂ by-product.

Table 1 .

At Cu:S atomic ratios of 1 :0.7 and 1 :1 .3, the XRD results showed the covellite phase (CuS) in all of the different samples. Covelite was also observed for treatment at 215°C at the Cu:S ratio of 1 :1 and 1 :1 .5. The results indicate that higher temperature treatment favours increased conversion between basic copper carbonate and sulphur. In each case, no elemental sulphur phase (S8) was detected in the sorbent by XRD when the sample was heated at 215°C.

For the above Cu:S atomic ratios, 100 % conversion of the basic copper carbonate to covellite was not achieved. The highest conversion to CuS achieved was for sample, Cu:S of 1 :1 .3 at 215°C, which showed 84 % conversion to covellite, which is close to the theoretical maximum conversion of 87 % based on sulphur content, with this slight difference likely a result of the approximations inherent within the XRD quantification employed. Extrapolation of the results reveals a ratio of Cu:S of ca. 1 :1 .6 should result in full conversion.

Since a treatment temperature of 180°C resulted in some conversion to covellite, the treatment temperature of 180°C was applied to samples of Cu:S (1 :1), under a nitrogen atmosphere for different amounts of time. The results are given in Table 2.

The decreasing carbon content with longer holding time at 180°C suggests increasing basic copper carbonate conversion while the decreasing sulphur content with longer holding time at temperature suggests there was increasing sulphur loss during the heat treatment process.

Table 2.

Table 3 sets out the conversion of the basic copper carbonate to covellite for the different atomic ratios at 180°C.

Table 3

Theoretical maximum conversion is based on the Cu:S ratio used. In a further example, a sorbent was prepared as above with a Cu:S atomic ratio of 1 :1 .5. A heat treatment at 250°C for 1 .5 hours gave full conversion of the basic copper carbonate to CuS with no S8 detected. Example 2. Heavy metal capture

Granules of absorbent precursor with a Cu:S atomic ratio of 1 :1 .5 were heated to 215°C for 1 .5 hours under nitrogen. The resulting sorbent (sieved to a 2.80 - 3.35 mm size fraction, volume 25 ml) was charged to a stainless steel reactor (21 mm ID). A flow of 100%vol natural gas was passed through a bubbler containing elemental mercury to allow the gas to pick up the mercury. The mercury-laden gas was then passed downwards through the reactor under the following conditions.

Pressure: 10 barg

Temperature 30°C

Gas flow 1 10.2 NL.hr-1

Contact time 8 seconds

Test duration 690 hours

Samples from the reactor inlet and exit were periodically analysed for mercury content by atomic fluorescence detection. The inlet gas had a mercury concentration at the inlet of about 1 ,100 mg/m ³ . The sorbent reduced the mercury content of the exit gas to below detectable limits throughout the test. At the end of each test the 25ml absorbent bed was discharged as 9 discrete sub-beds which were ground completely and analysed by acid digestion/ICP-OES to determine total mercury content. The amount of mercury captured by each absorbent bed is shown in Table 4.

Table 4

Cu:S = 1:1.5

215° C, 1.5 hrs

Bed 1 (inlet) 24141

Bed 2 12104

Bed 3 8101

Bed 4 3195

Mercury

Loading, Bed 5 360

ppm wt%

Bed 6 353

Bed 7 225

Bed 8 20

Bed 9 (exit) 27 The test indicates that the sorbent was effective in trapping mercury in the gas stream. Comparative Example

A mixture of un-sulphided basic copper carbonate sorbent precursor granules prepared according to WO2009/101429 and sulphur powder at a Cu:S atomic ratio of 1 :1 was heated to 215°C for 1 .5 hours in nitrogen flow. Alternatively, the basic copper carbonate sorbent precursor granules were calcined in air at 265°C and were held at this temperature for 4 hours to transform the basic copper carbonate therein to copper(ll) oxide. After cooling, a mixture of the copper oxide granules and sulphur powder at a Cu:S atomic ratio of 1 :1 was heated to 215°C for 1 .5 hours in nitrogen flow.

These heat treated material were analysed by XRD to quantify the weight percentage of each phase. The conversions to covellite, CuS, were 32 % and 22 % for the basic copper carbonate and copper oxide granules respectively. This result indicates (i) that the mixed particulate powder method of the present invention is more effective in forming covellite at 215°C than using pre-shaped granules, and (ii) that basic copper carbonate is more suitable for preparing sorbents that copper oxide.