WO2007017666A1

WO2007017666A1 - Process for treating a bone char material

Info

Publication number: WO2007017666A1
Application number: PCT/GB2006/002952
Authority: WO
Inventors: James Russell
Original assignee: Brimac Carbon Services Limited
Priority date: 2005-08-08
Filing date: 2006-08-07
Publication date: 2007-02-15

Abstract

The invention provides a process for the treatment of bone char to give a novel bone char product, which has utility in the removal of contaminants from fluids. The products are especially useful in the removal of radionuclides from aqueous waste.

Description

PROCESS FOR TREATING A BONE CHAR MATERIAL

The present invention relates to an adsorbent product, to a process for the preparation of such a product, and to a plurality of uses of such a product.

TECHNICAL BACKGROUND AND PRIOR ART

Many harmful contaminants exist in the environment, either because they are present naturally (for example, ground water in Bangladesh is known to have very high levels of arsenic), or as the result of the activity of man. An increasing awareness of the effects on the health of human beings and other organisms of such pollutants has given rise to the need to reduce their level in the environment, both by eliminating the sources of pollution and remedial treatment of contaminated areas.

A particular problem is the contamination of groundwater. In many parts of the world, groundwater forms the bulk of the potable water available to the local population. Any contaminants present in the groundwater therefore present a serious risk to health through ingestion. The problem is further exacerbated by the fact that once pollutants have entered the groundwater, they tend to spread therein by various mechanisms including diffusion and tidal flow.

Remedial removal of contaminants from contaminated groundwater is far from easy, as by definition groundwater is present in subterranean aquifers. Approaches fall into two broad categories:

i. Pump-and-treat: groundwater is extracted from below the water table, treated to remove contaminants, and the water returned;

ii. In situ methods, such as "trenching", wherein a trench filled with adsorbent is placed within the path of flow of the groundwater, such that water passing through the trench has contaminants removed.

Metals are one type of contaminant associated with groundwater, and the toxicity of heavy metals such as lead, cadmium and mercury is well documented. Of special concern is contamination by uranium and other radioactive materials, as these are harmful in very low concentrations, and persist in the environment for many years. Bone char (also known as bone charcoal or bone black) is the product obtained from the calcining of animal bones. Its chemical constituents are principally apatite (or hydroxyapatite) and carbon. Bone char has a very high surface area to weight ratio, and for this reason it is a very good adsorbent of many materials present in aqueous systems. The principle use of bone char is the removal of coloured impurities in the sugar refining process.

Bone char has previously been shown to be effective in reducing the level of contaminants in water. US4,902,427 discloses a filter cartridge for removing heavy metals from water, comprising a bone char impregnated filter. The cartridges are stated to be useful in the removal of cadmium, lead and mercury from water passed through them.

US6,428,695 discloses an in situ method for the removal of uranium from contaminated groundwater. Trenches are dug through the flow path of groundwater, and permeable reactive barriers are installed. A barrier consisting of bone char and iron oxide is shown to be superior to a barrier consiting of bone char alone in the removal of uranium from contaminated water near an abandoned uranium upgrader.

The need remains for new adsorbents for use inter alia in the removal of contaminants, in particular radionuclides from water.

The present invention seeks to overcome / address the problems associated with the prior art.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a process for the preparation of a bone char product of the invention comprising the steps of: a. converting at least an amount of the carbon present in bone char to give a volatile product; and b. separating said volatile product from the solid residue.

According to a second aspect, the invention relates to a bone char product having a reduced carbon content relative to virgin bone char. According to a third aspect, there is provided a product obtainable by the process of the invention.

According to a fourth aspect, there is provided the use of a bone char product of the invention for removing a contaminant from a fluid.

According to a fifth aspect, there is provided a process for the removal of a contaminant from a fluid, comprising bringing into association a bone char product of the invention with a contaminated fluid, to give a fluid having reduced level of contaminant and a bone char product loaded with contaminant.

DETAILED DESCRIPTION OF THE INVENTION

BONE CHAR

The bone char products of the invention may be manufactured using any type of bone char. Suitable bone chars are those made from animal bones. Particularly suitable bone chars are those made from cattle, sheep, pig, horse and fish. Bone char made from cattle bones is very highly preferred.

The bone char products of the invention may be prepared from virgin bone char (i.e. freshly manufactured bone char), or spent bone char (bone char that has been used previously for some other purpose, such as decolourising sugar).

The process used for the preparation of the bone char starting material is not critical to the invention, and any standard preparation method may be used. Typically, bone char is produced by heating the animal bones in a reducing atmosphere. A suitable method for producing bone char starting material is to heat bone grist in an oxygen-deficient atmosphere for 7 hours at a temperature of 700 ⁰C. Bone char produced in this way has a carbon content of between 9.5 and 11.5 %.

The skilled person will be aware that virgin bone char has a carbon content of about 10 % w/w. The particle size of the bone char starting material is not critical to the success of the process, and any commercially available grade may be employed. The preferred particle size is "8/60 grade" (particle size 2.3mm to 0.25mm). More preferred is "20/60 grade" which has a particle size of 1 mm to 0.25 mm.

PROCESS

In a broad aspect, the bone char starting material is converted to the bone char product of the invention by converting at least an amount of the carbon present in the bone char starting material to a volatile product, and separating the volatile product from the bone char product.

As used herein, the term "volatile product" means a chemical compound having a boiling point lower than that of elemental carbon.

Preferred volatile products include hydrocarbons, that is compounds of carbon and hydrogen. Particularly preferred volatile products are methane and ethane.

In an alternative preferred embodiment, the volatile products are oxides of carbon. Preferred oxides of carbon are carbon monoxide, carbon dioxide or a mixture of the two.

The conversion of the carbon present in the bone char starting material to the volatile product may be accomplished by any one of a number of means.

In a preferred embodiment, the bone char starting material is brought into association with a reducing agent. As used herein, the term "reducing agent" means a chemical compound or combination of compounds capable of reacting with elemental carbon present in the bone char starting material to give a product wherein the oxidation state of the carbon is negative.

A suitable reducing agent is hydrogen. In this case, the volatile product will be a mixture of hydrocarbons.

In an alternative preferred embodiment, the bone char starting material is brought into association with an oxidising agent. As used herein, the term "oxidising agent" means a chemical compound or combination of chemical compounds capable of reacting with the elemental carbon present in the bone char starting material to give a product wherein the oxidation state of the carbon is positive.

Suitable oxidising agents include halogens and their oxides, peroxides (especially hydrogen peroxide), and oxidising transition metal compounds, such as chromates (Vl) an manganates (VII).

A very highly preferred oxidising agent is oxygen. This may be used as pure oxygen, or may be mixed with other oxidising or diluent gases.

As used herein, the term "diluent gas" refers to any gas which does not significantly participate in a reaction with the carbon present in the bone char starting material under conditions wherein the same carbon reacts with oxygen.

Typical diluent gases include nitrogen, the noble gases, carbon monoxide, carbon dioxide and water vapour.

A particularly preferred source of oxygen is air.

In the embodiment wherein the oxidising agent comprises oxygen, the volatile products will comprise carbon monoxide, carbon dioxide or a mixture of the two.

The conversion of the carbon present in the bone char starting material to the volatile product may be achieved in a number of ways.

In a preferred embodiment, the bone char starting material may be treated with a solution comprising an oxidising or reducing agent. It may be necessary to heat, agitate or otherwise stimulate the mixture so formed, so as to promote the conversion of the carbon to volatile product.

In an alternative highly preferred embodiment, the bone char starting material may be treated by bringing it into association with a gaseous oxidising agent or reducing agent. Again, it may prove necessary to promote the reaction by means of heat and/or agitation.

More preferably, the bone char starting material is heated in oxygen containing gas at above 300 ⁰C, more preferably at above 400 ⁰C, more preferably at above 500 ⁰C, more preferably at above 600 ⁰C, more preferably between 500 and 800 ⁰C, more preferably between 600 and 800 ⁰C, more preferably between 600 and 700 ⁰C.

SEPARATION

In a broad aspect, the volatile products may be separated from the bone char products by any suitable physical separation method. A suitable separation method is to allow the volatile products to diffuse out from the bone char product.

Such diffusion can be facilitated by heating, for example heating above the boiling point of the volatile product. The passage of gas through the bone char product can also assist in freeing it from volatile product.

In a particularly highly preferred embodiment, bone char starting material is heated in an oxygen-containing gas, to give bone char product and a volatile product comprising oxides of carbon. The bone char product is separated from the volatile product by diffusion.

In this embodiment, it is preferable that the bone char starting material is heated in a stream of oxygen containing gas. By this means, the oxidising agent and carbon are effectively brought into contact, and the volatile products are swept out of the bone char product.

A particularly convenient means for conducting the process of the invention is in a rotary calciner. A suitable apparatus is shown in Figure 1. Oxygen containing gas (2) is passed through the rotating calciner (3) containing bone char starting material (6) fed continuously from a hopper heated by a vertical kiln (7). The calciner (3) is held at a temperature appropriate to conducting the reaction at a reasonable rate. The rotation of the calciner (3) ensures that the bone char starting material is evenly mixed, and ensures a more uniform product (1). Flue gas is discharged at (5), and may be used to heat bone char feed (4). Evolved gas (8) may be collected for recycling.

A particular advantage of using such a rotary calciner is that the process of the invention may be operated in a continuous fashion, with bone char starting material being introduced into one end of the calciner, and bone char product being discharged from the other. To assist in this, the calciner may be inclined (as shown in Figure 1) so that the bone char particles proceed from inlet to outlet of the calciner under the influence of gravity.

Preferably, the rotary calciner is inclined at an angle of less than 10 °. More preferably, the rotary calciner is inclined at an angle of less than 1 °. Most preferably, the rotary calciner is inclined at an angle of about 0.1 °.

Preferably, the rotary calciner is rotated at a rate of less than 10 revolutions per minute. More preferably, the rotary calciner is rotated at a rate of less than 5 revolutions per minute. More preferably, the rotary calciner is rotated at a rate of about 3 revolutions per minute.

Preferably, bone char is introduced into the calciner at a rate of less than 100 kg per hour. More preferably, bone char is introduced into the calciner at a rate of less than 50 kg per hour. More preferably, bone char is introduced into the calciner at a rate of about 30 kg per hour.

Preferably, airflow through the calciner is maintained at less than 50 litres per second. More preferably, airflow through the calciner is maintained at less than 25 litres per second. Most preferably, airflow through the calciner is maintained at about 14 litres per second (0.5 cubic feet per second).

In a very highly preferred embodiment, the rotary calciner is rotated at a rate of about 3 revolutions per minute and inclined at an angle of about 0.1 °, bone char is introduced into the calciner at a rate of about 30 kg per hour and airflow through the calciner is maintained at about 14 litres per second (0.5 cubic feet per second),

When the process of the invention is conducted in a rotary calciner, then the calciner may be fired directly or indirectly. It is preferred that the rotary calciner is fired directly.

CONTROL OF PROCESS

In order to control the process so as to achieve the correct proportion by weight of carbon, it may optionally be necessary to control the process. This may preferably be achieved in one of several ways: Metering - the required amount of oxidising or reducing agent is calculated based on the amount of carbon in the bone char starting material, and the amount desired in the final product. Only this amount of oxidising or reducing agent is allowed to react with the bone char starting material;

Monitoring - the amount of carbon present in the bone char is measured periodically through the course of the reaction, and when it has reached the desired level, the reaction is stopped;

Timing - once the rate of reaction under particular conditions has been established, it will generally be most convenient to conduct the process for a set amount of time (other variables such as flow rate of oxygen-containing gas being constant).

In the particular embodiment referred to above, wherein the bone char starting material is held in a rotary calciner, a convenient means for controlling the reaction is to adjust the speed of rotation and angle of incline of the rotary calciner such that average residence time of the bone char starting material in the calciner is constant.

PRODUCT

The products of the invention are characterised in that they have a carbon content lower than that of virgin bone char. In particular, the products of the invention have a ratio of apatite to carbon that is higher than the ratio of apatite to carbon in conventional virgin bone char. The ratio of apatite to carbon in virgin bone char is typically 10:1. Preferably, the bone char of the invention has a ratio of apatite to carbon greater than 10:1. More preferably, the bone char of the invention has a ratio of apatite to carbon greater than 15:1. More preferably, the bone char of the invention has a ratio of apatite to carbon greater than 20:1.

Preferably, the products of the invention have a carbon content of less than 8.5 % w/w. More preferably, the products of the invention have a carbon content of less than 8.0 % w/w. the products of the invention have a carbon content of less than 7.5 % w/w. More preferably the products of the invention have a carbon content of less than 7.0 % w/w. More preferably the products of the invention have a carbon content of less than 6.5 % w/w. More preferably the products of the invention have a carbon content of less than 6.0 % w/w. More preferably the products of the invention have a carbon content of less than 5.5 % w/w. More preferably the products of the invention have a carbon content of less than 5.0 % w/w. More preferably the products of the invention have a carbon content of less than 4.5 % w/w. More preferably the products of the invention have a carbon content of less than 4.0 % w/w. More preferably the products of the invention have a carbon content of less than 3.5 % w/w. More preferably the products of the invention have a carbon content of less than 3.0 % w/w. More preferably the products of the invention have a carbon content of less than 2.5 % w/w. More preferably the products of the invention have a carbon content of less than 2.0 % w/w. More preferably the products of the invention have a carbon content of less than 1.5 % w/w.

Preferably, the products of the invention have a carbon content of at least 0.5 % w/w. More preferably, the products of the invention have a carbon content of at least 1 % w/w. More preferably, the products of the invention have a carbon content of at least 1.5 % w/w. More preferably, the products of the invention have a carbon content of at least 2.0 % w/w. More preferably, the products of the invention have a carbon content of at least 2.5 % w/w. More preferably, the products of the invention have a carbon content of at least 3.0 % w/w. More preferably, the products of the invention have a carbon content of at least 3.5 % w/w. More preferably, the products of the invention have a carbon content of at least 4.0 % w/w. More preferably, the products of the invention have a carbon content of at least 4.5 % w/w.

In a very highly preferred embodiment, the bone char products of the invention have a carbon content of between 1.5 and 6.5 % w/w, more preferably between 4.0 and 6.0 % w/w, most preferably between 3 and 5 % w/w.

USES

The bone char products of the present invention are useful in the removal of the contaminant from a fluid.

"Fluid" as used herein refers to a gas or a liquid.

"Contaminant" as used herein refers to a material that it is desired to remove from the fluid, for example because it is toxic, or alternatively valuable. The contaminant may be present in the fluid as a solution, suspension, emulsion, slurry or any other form. A preferred fluid is sugar solution. In this case, the contaminant is coloured impurities.

An alternative preferred fluid is water. The bone char products of the invention show particular utility in this case, because of the diverse range of contaminants that can be removed. These include:

• metals, such as aluminium, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, silver and zinc;

• anions, such as fluoride and chloride; • organic compounds, such as pesticide residues;

• endocrine disrupters such as phthalates, bisphenol A, alkylphenols, parabens and phytoestrogens;

• radionuclides, such as ²⁴¹Am, ¹⁰⁹Cd, ¹³⁹Ce, ⁶⁰Co, ¹³⁷Cs, ⁵⁴Mn, ¹¹³Sn, ⁸⁵Sr, ⁹⁹Tc, ⁸⁸Y ⁶⁵Zn, and uranium especially ²³⁵U.

As used herein, the term "endocrine disrupter" refers to any material having the ability to mimic a hormone in an animal.

The products of the invention are particularly suitable for adsorption of radionuclides, and in particular are more effective than known bone chars.

Thus, the bone char products of the present invention find utility in the treatment of potable water, sewage, and the water used in aquaculture (fish farms).

The products of the invention, once they have adsorbed contaminant, are referred to herein as "loaded". The loaded products may be stored. To this end, the products of the invention may be subjected to one or more subsequent treatment steps to render them in a form suitable for storage.

Optionally, the loaded product may be encapsulated. The term "encapsulated" as used herein refers to a process whereby loaded product is embedded, coated, or otherwise sealed within another material to prevent it interacting with the environment. Examples of suitable encapsulating materials are glass, plastics materials, concrete, ceramics and metals. A preferred method of encapsulation is vitrification, i.e. glass is used as the encapsulating material. In a typical vitrification method, loaded product is mixed with frit and fused. Suitable methods are described in US5,873,675; US5,603,684; and US 5,662,579. A general summary is provided in the EPA Handbook entitled "Vitrification Technologies for Treatment of Hazardous and Radioactive Waste".

Alternatively, the contaminant may be a material that it is desirable to recover from the fluid, i.e. a valuable material.

The valuable material may be present in the fluid as a solution, suspension, emulsion, slurry or any other form.

Examples of valuable materials include:

• precious metals, particularly silver, gold, platinum, palladium and rhodium; • fissile material, particularly uranium (present e.g. in seawater).

The terms "contaminant" and "valuable material" are not necessarily mutually exclusive, e.g. uranium may be both a contaminant and a valuable material.

Subsequent to adsorbing valuable material from a fluid, the loaded products of the invention may be treated to recover valuable material. For example, loaded product may be washed with an aqueous medium to give a solution of valuable material.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be described in further detail by way of example only with reference to the accompanying figures in which:-

Figure 1 is a diagram of an apparatus suitable for conducting a process of the invention.

The present invention will be described in more detail with reference to the following non- limiting examples.

EXAMPLES

Example 1 An indirect calciner was fitted with a paddle feeder, disturber cage and 2 inch open weir and air cooler for the test. The slope was set at 0.5 degrees and the revolution speed at 3 rpm. A feed rate of 30 kg per hour was maintained for the duration of the test.

Air was passed through the rotating kiln at 0.5 ft³ per second. The feed hood damper was fully opened. The product hood and bypass dampers were closed. After running for five hours, a pale blue carbon monoxide flame appeared about one-third of the way into the kiln. Increasing the purge air by closing the burner inlet damper and the direct calciner exhaust had no effect on combustion indicating that airflow was adequate.

Gas flows were initiallly adjusted to maintain a nominal temperature of 600 ⁰C. At these conditions there was evidence of overcalcining and the gas flow were reduced after 2 hours 50 minutes to lower the material temperature.

Samples were taken every 30 minutes. The loss on ignition and percentage of carbon present were evaluated.

A sample taken after two hours had a loss on ignition (LOI) of 7.6 %. Based on this result it was decided to increase the calcining temperature to a nominal 700 ⁰C. The test was terminated after six hours. The maximum temperature achieved was 690 ⁰C.

TEST RESULTS ON SAMPLES

Table 1

Example 2

31 m³ of 20/60 bone char was received for decarbonisation. This was stored in clean water in the "Spent Silo". Transfer to the feed hopper was facilitated by recycled process water that had previously been neutralised.

Initial settings were: kiln temperature 700 ⁰C (final third of kiln)

Char flow rate 750 litres per hour

This equated to a residence time of 2 hours, dependent on kiln rotation speed RESULTS

Table 2

All publications mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in chemistry or related fields are intended to be within the scope of the following claims.

Claims

1. A process for the preparation of bone char product comprising the steps of: a. converting at least an amount of the carbon present in bone char to a volatile product; and b. separating said volatile product from the solid residue.

2. A process according to claim 1 wherein the volatile product comprises an oxide or oxides of carbon.

3. A process according to claim 1 or 2 wherein step a) is conducted by heating bone char in an oxygen-containing gas.

4. A process according to claim 3 wherein the heating step is conducted at a temperature of between 500 and 800 ⁰C.

5. A process according to any one of claims 1 to 4 wherein steps a) and b) are conducted in a rotary calciner.

6. A bone char product having a reduced carbon content relative to virgin bone char.

7. A bone char product according to claim 6 comprising less than 6 % by weight of carbon.

8. A bone char product according to claim 7 comprising between 3 and 5 % by weight of carbon.

9. A bone char product obtainable by a process as claimed in any one of claims 1 to 5.

10. Use of a bone char product as claimed in any one of claims 6 to 9 in a process for the removal of a contaminant from a fluid.

11. Use according to claim 10 wherein the contaminant comprises a radionuclide.

12. Use according to claim 11 wherein the radionuclide is at least an isotope of uranium.

13. Use according to claim 10 wherein the fluid is sugar solution.

14. Use according to claim 10 wherein the contaminant is an endocrine disrupter.

15. Use according to claim 10 wherein the contaminant is a heavy metal.

16. Use according to claim 10 wherein the contaminant is a precious metal.

17. A process for the removal of a contaminant from a fluid, comprising bringing into association a bone char product as claimed in any one of claims 6 to 9 with a contaminated fluid, to give a fluid having reduced level of contaminant and a bone char product loaded with contaminant.

18. A process according to claim 17 comprising a further step of encapsulating bone char product loaded with contaminant.

19. A process according to claim 18 wherein the encapsulating material is a glass.