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Industrial Processes

Microwave heating of samples

   
 
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The partners in the project have developed a microwave-based system for rapidly dissolving, or digesting, samples in solution in laboratories.
The digester is faster and more energy-efficient than the usual method of heating the solution on a hotplate or in a water bath. The main novelty is the ability to simultaneously heat four vessels containing solutions. Multiple heating should speed up laboratory work and be less polluting.

One of the routine tasks in chemical laboratories is the preparation of samples for analysis by dissolving material in solution by heating, a process often known as digestion. The process is also known as mineralisation, particularly when organic samples of heavy metals are prepared. Traditionally samples are prepared by placing a beaker containing solution and the material to be dissolved on a hotplate or in a water bath. The vessel has to be heated, typically for five minutes.
The partners in the Multidigester 4000 project decided to investigate the use of microwaves for more efficient and controllable heating of vessels to improve the efficiency of sample preparation. An important aim of the project was to produce a unit that could simultaneously heat four open vessels of solutions and substances. The design directs all the microwave energy into the vessels, increasing the energy efficiency of the equipment. In addition, the equipment is automatic in order to simplify use.

Better focus on heating

The use of microwaves for heating in the laboratory is not new. There is already a piece of equipment which can heat a single open vessel, and there is also an oven used with closed vessels in which the pressure rises to improve the dissolving of materials. However, equipment based on domestic microwave ovens produces a poorly-controlled distribution of microwaves, leading to uneven heating and poor control over the quality of the samples being prepared.
The Multidigester partners decided to develop a specialised unit for laboratories. The partners included Soled Industries, which manufactures a range of laboratory equipment, the instrumental analytical equipment department of Gerhard-Mercator-University of Duisburg in Germany, and Puls Plasma Technik of Germany, which produces pulsed energy equipment such as ultrasonic machines to treat kidney stones.

Power splitting

The unit developed by the partners is based on a single 2.45 GHz microwave generator, or magnetron. The microwaves are directed into a square-section beam splitter to split the single flow of energy into four equal and symmetrical beams, which are then directed to four heating chambers.
Test tubes or other vessels containing sample solutions are placed in the four chambers for heating by the microwave beams, with nearly all of the microwave energy passing into the sample and solution. The efficiency of heating means that simple samples can usually be prepared within one minute, rather than the five minutes needed when using a hotplate or water bath. The digester can produce between 10 W and about 200 W, depending on the samples being prepared.
The design could be developed for laboratories that prepare large numbers of samples by adding further magnetrons for further multiples of four heating chambers.
The heating chambers are in the form of openings in the top of the unit, into which the sample vessels are placed. This allows a laboratory to prepare samples in a range of vessels. The design also carries out a heating cycle automatically, under the control of a microprocessor that can be programmed by the user.

Trials show faster results

Puls Plasma Technik produced an initial version of the machine for testing at Gerhard-Mercator-University and five further units for wider trials. Faults that appeared during trials included small cracks in welds which produced sparking, and a movement of the Teflon base of some heating chambers, leading to irregular heating. The faults were corrected, in the latter case through the use of electrically insulating material to fix the bases.
The partners enlisted the help of a number of chemical laboratories, including Shell Research in Amsterdam, Solvay in Brussels, Merck in Darmstadt, British Steel in Teesside and INA in Paris. Trials were also carried out by the project partner, the Gerhard-Mercator-University.
The trials have shown that the time needed to dissolve heavy metals can be cut from a normal 120 minutes to 10 minutes, and gold is dissolved in 7 minutes rather than the 60 minutes needed using conventional heating.

Practical problems

Unequal heating of the four chambers on a machine was a problem with the prototypes. The reasons were because of variations of the flatness of the bases of the vessels placed in the chambers, and because of poor positioning of the microwave guides. The first problem has been solved by the production of sample vessels with flat, polished glass disc bases. Better manufacturing control during commercial production will solve the positioning problem.
There was also a need to eliminate any leakage of microwaves, particularly when not all four chambers are in use. Since 1996, European legislation specifies a leakage of 0.5 milliwatts/cm2. It was found that liquid in the sample vessels emitted microwaves, and that the microwaves escaped if the liquid level was too high or boiled.
This problem has now been solved, say the partners, however, work is still under way on solving an additional problem of optimising the splitting of the energy from the magnetron into four microwave beams of equal power. This is particularly important to ensure good control of sample preparation. The use of a machine for several hours also raised questions about the risk of overheating, although the partners do not believe this will be a concern with future commercial versions of the design.

Wide market interest

The prototype unit has been shown at two exhibitions and generated considerable interest. After evaluating a unit in trials, Merck expressed an interest in participating in the commercial development of the design. The system of directing the microwaves to the vessels is the subject of a patent application.
The partners see a large potential market in chemical laboratories in many industrial sectors. The design could prepare samples of a wide range of materials, including soil, detergent, ore, industrial waste, sand, water, dye, and geological samples. The steel sector would be just one of many potential markets. The partners believe a commercial stage could be reached by the middle of 1997.
In future, an automation system could be developed for use with the digester, for even greater laboratory efficiency.

 

 

Project Title:  
Multidigester

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: CECA 7210-GD-107

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