CA1262167A - Method and apparatus for thawing of ore or concentrate contained in a railroad car - Google Patents

Abstract

Abstract of the Disclosure:

A method and an apparatus for thawing ore or concentrate contained in a railroad car is disclosed.
Current conducting electrodes are inserted in the ore or concentrate in the railroad car, and a voltage source is applied to the electrodes to cause a current of sufficient value to flow through the electrodes to produce heat and melt the ice around the electrodes.

Description

METHOD AND APPARATUS FOR THAWING OF ORE OR CONCENTRATE
CONTAINED IN A RAILROAD CAR
This invention relates to thawing of ore or concentrate which freezes in railroad cars during transportation in cold weather.
Transportation of wet ore or concentrate in railroad cars in cold weather results in freezing of the water in the ore or concentrate and this ice must be meltsd before unloading of the railroad cars. Thawing of ore or concentrate is normally done by entering the cars into so called thaw sheds which are large enough to accommodate a number of cars and heating up the cars using steam heaters or direct-fired hot-air heaters. This operation is expensive because a lot of energy is-lost in heating the shed and the cars.
It is therefore the object of the present invention to provide a new method of thawing ore or concentrate wherein the heating energy is applied directly to the ore or concentrate in the cars instead o~ being applied to the aars to heat up the material inside the cars.
The method in accordance with the present invention comprises tha steps of inserting current conducting electrodes in the ore or concentrate in the railroad car, and applying a voltage source to the electrodes to cause a current of sufficient value to flow through the electrodes to produce heat and melt the ice around the electrodes.
The temperature o~ the ore around the electrodes .~

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-2-or the amount of current flowing khrough the electrodes is preferably monitorad to limit the ~low of current through each electrode so as to prevent boiling of the melted ice around the electrodes.
The system for carrying out the method in accordance with the present invention comprises means for inserting current conducting electrodes through the ore or concentrate in the railroad car, and means for connecting a voltage source to such electrodes for flowing an electric current of sufficient amplitude through the electrodes to produce heat and melt the ice around the electrod~s.
The voltage sourcP preferably comprises a three-phase transformer with a star connected secondary winding so as to allow connection of the neutral common point of the secondary winding of the transformer to the car structure.
Thermocouple~ may be inserted into the ore or into a hollow electrode to monitor temperature o~ the ore around the electrodes. Similarly, current transformers may be connected to tha electrodes to monitor the current flow into the electrodes. The output of the thermocouples and o~ the current transformers may be used to control the f~ow of current through the electrodes to prevent boiling of the melted ice around the electrodes.
~ datalogger may be provided to register temperature, current and voltage data. This data may also be stored into a microprocessor for analysis or treatment.

~;2~

-3-The invention w~ll now be dlsalosed, by way o~
example, with reference to the accompanying drawings in which:
Fiyure 1 i]lustrates a schematic diagram o~ a system which may be used ~or thawing of ore or concentrate;
Figures 2 a and b illustrate location of the electrodes, current trans~ormers and thermocouples in a railroad car;
Figures 3 and 4 illustrate the variation o~
current in selected eleotrodes versus time; and Figure 5 illustrates the variation of temperature and the energy consumed in a typical electrode versus time.
Referring to Figure 1, there is shown a schematic diagram o~ a system which could ba used ~or thawing ore or concentrate lo contained in a railraod car 12. A~ shown in Figures 2a and 2b, three rows of electrodes 14 are driven into the ore up to a few inches,from tha bottom o~ the car. The electrodes are connected to the secondary 2Q windings Ts of one or several three-phase trans~ormers depending on the electrical load and their connection is evenly distributed ~o as to balance the load hetween the phases ~A, ~B and ~C. The secondary windings Ts of the transformers are preferably skar connected so as to permit connection o~ the neutral N to the car structure.
Current transformers 16 may be pro~ided for monitoring the current flowing through selected electrodes. Thermocouples 18 are also inserted at d ~

selected locations into the ore to monitor the temperature of the ore at such location. The voltage and current parameters are detected by transducers 20 and 22, respectively, and fed to a datalogger 24. The thermo-couples are also connected to the datalogger in order toregister all the data at various time intervals during thawing. A microprocessor 26 may also be provided for registering all data on magnetic discs for analyses or treatment.
Prior to making full thawing tests, a few preliminary tests were done to determine where the heat was generated during thawing. The concentrate tested contained 12.7% of water, 24.9% Cu, 0.12%Pb, 0.7% Zn, 0.1 oæ. per ton Au and 2.35 oz. per ton Ag. It has been found that the main heat source is due to the contact resistance between the electrodes and the ore which surrounds the electrodes. This resistance varies between a fraction of an ohm to a few ohms from one electrode to the other because the ore is not uniformly distributed th~oughout the car and thus the ore contact area with each eleatrode is not the same.
It has been found that the ore itself is a good conductor and that very little heat was generated by the flow o~ current through the ore. It was also found that the contact resistance between the car and the ore was also low. Furthermore, the heat generated by suoh contact resistance is dissipated on a large su~ace area which tends to minimize its e~ect.

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The results of a ~ull scale test on thawing of a concentrate o~ the above composition contained in a railroad car are illustrated in the following Tables wherein Table 1 gives the temperature measured by thermo-couples located at selected locations TJl-TJ16 as indicated in Figure 2b of the drawings at various time intervals, and Table II gives the currents I measured in phases ~A and 0C and I4-I9 measured in selected electrodes as also indicated in Figure 2b. The voltage applied, power dissipated, and electrical energy consumed as the test proceeded are also illustrated in Table II.

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TAB L~ I

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' As shown in Table II and illustrated in Figures 3 and 4, the current in the electrodes increases (2 to 3 times its initîal value) as the ore heats up around the eleatrodes. This is due to an improved contact between the s electrodes and the ore as thawing proceeds and also to the improvement in the conductivity of the ore as a funckion of increase in temperature. The electrodes which make better contact right at the beginning o~ thawing have a tendency to experience a faster current incrQase than the others.
Consequently, more heat is developed around these eleatrodes which increasss their conductivity even more.
This may be observed by comparing currents I4 and I9 in Figure 3 and currents I6 and I7 in Figure 4. If the above process continues without control, there will be enough heat developed around the electrodes having lower contact resistance to cause the water around the electrodes to boil. This phenomenon must be avo~ded because the energy re~uired to vaporize water is useless. This is illustratad in Figure 4 wherein the temperature at TJ5 exceeded 100C
shortly after 2~4 hours of operation. If boiling is continued, it will dry the ore around the electrode and this may eventually cause a short-circuit. Thus, it is preferable to limit the current in the electrodes to prevent ~oiling and allow a more uniform thawing of the ore. This may be done by means of a thyristor located in the ~eed of each electrode. The current through each electrode may be controlled khrough a feedback circuit connected to the ~iring gate of the thyristor and lt;7 , .

responsive to a thermocouple measuring the temperature of the ore around the electrode or to a current transfomer measuring the aurrent flowing through the electrode.
In the above test, the average temperature of the ore was raised from -5C to +15C in about 7 hours. The energy required was about 1.1 MWh to thaw about 80% of a car containing 96 tons of ore. Based on theoretical calculation, it was found that the efficiency of the test was about 90%. This efflciency would have been increased if boiling around the electrodes had been controlled.
Subse~uent tests have proven that it was possible to insert electrodes in frozen ore by the combined action of pressure and heat generated by the electrode~ To that effect, a group of sixteen 3/4 in electrodes were installed on a supporting structure which was held above a car of frozen ore by chain blocks. The supporting structure was loaded with about 7,000 pounds of cement blocks making up a }oad of about 430 pounds per electrode.
A three phase voltage of 100 V was applied on the eleckrodes upon contact with the ore. The temperature was about -5~C at the surface of the ore, -3C inside the ore and the ambient air temperature -7C. Thawing was very rapid near the point of the electrodes thus facilitating their insertion into the ore. After 35 minutes, ~he electrodes were fully inserted into the ore.
Although the invention has been disclosed with refere.nae to a preferred embodiment, it is to be understood that it is not limited to such embodiment and that other alte.rnatives are also envisaged within the scope of the ~`ollowing claims.

Claims (5)

1. A method of thawing ore or concentrate contained in a railroad car comprising the steps of:
a) inserting current conducting electrodes in the ore or concentrate in the railroad car, b) applying a voltage source to the electrodes to cause a current of sufficient value to flow through the electrodes to produce heat and melt the ice around the electrodes, and c) monitoring the temperature of the ore around the electrodes or the amount of current flowing through each electrode to limit the current flow through the electrodes so as to prevent boiling of the melted ice around the electrodes.

2. A system for thawing ore or concentrate contained in a railroad car comprising:
a) means for inserting current conducting electrodes in the ore or concentrate in the railroad car;
b) means for applying a voltage source to the electrodes to cause sufficient current to flow through the electrodes to produce heat and to melt the ice around the electrodes, and c) means for monitoring the temperature of the ore around the electrodes or the current flow through each electrode to limit the current flow through the electrodes so as to prevent boiling of the melted ice around the electrodes.

3. A system as defined in claim 2, wherein the voltage source comprises a 3-phase transformer having a star-connected secondary winding and means for connecting the neutral common point of said secondary winding to the railroad car.

4. A system as defined in claim 2, wherein said monitoring means comprise thermocouples located in the ore around the electrodes or in the electrodes for monitoring the temperature of the ore and means responsive to said thermocouples for controlling the current flow through the electrodes.

5. A system as defined in claim 2, wherein said monitoring means comprises current transformers connected to said electrodes for monitoring the amount of current flow in said electrodes and means responsive to said cur-rent transformers for controlling the current flow in the electrodes.