Table of Contents
In 1965 Hoogovens was looking for additional capacity for burden preparation in the order of 2.5 to 3 million metric tons per annum of some kind of optimal blast furnace ore feed. In principle four solutions offered themselves for further investigation:
- purchase of screened ore and/or run of mine ores with the provision of a corresponding ore screening capacity on the blast furnace site;
- manufacture of sinter;
- purchase of pellets;
- manufacture of pellets.
Selection of the equipment parts
A survey of existing installations revealed a great number of alternative solutions out of which a choice had to be made:
a. mixing of the ores from bins or the use of mixing beds;
b. dry or wet grinding before or after mixing;
c. open or closed grinding;
d. disc or drum balling;
e. grate kiln or straight grate installations for the induration.
The choosing was done comparing the results of numerous tests, ordered by Hoogovens at various pilot stations.
When mixing of the ores before grinding it could be feared that the soft ores would be overground and the hard ores stay too coarse.
In practice, however, the grindability of rich ores does not differ much, so that a good narrow screen analysis of the ground ore can be obtained when grinding the mixture. The pellets are made of ore mixtures and for an efficient blast furnace operation it is necessary to produce pellets with a constant chemical composition.
Most pelletizing plants are situated near the mine and obtain their ore from the concentrator.
The ore is delivered as a slurry and consequently grinding is performed wet. After grinding the slurry is pumped to vacuum filters where the water is removed. The filter cake is transported to the balling section for making green balls.
This system has some drawbacks:
a. the control of the moisture content is difficult and depends on the nature of the ores and the degree of clogging of the filter cloth. Especially for earthy ores the remaining moisture content of the filter cake is high and variable. Correction by absorbing the water in bentonite is possible, but this is a costly solution, especially in Europe.
b. the ball consumption is high with wet grinding, caused by erosion and corrosion of the balls.
c. it is probably difficult to grind a self fluxing mixture.
Three systems have proved to be qualified for balling: discs, cones and drums. In a drum circuit, a screen and a recirculation system for the screen out of small balls are necessary with cones and discs the rotating movement itself causes a classification.
The straight grate
On a straight grate drying, heating and cooling is combined on the same machine. The relation between the available time for the different stages of the process is fixed.
As a disadvantage is often considered the short indurating time, especially for the lower pellets of the bed.
An advantage is the immobility of the pellets during the heat treatment, so that the value of a preheat strength represents no limitation to the process.
The grinding section consists of three lines. The three mixture bins of 600 m³ (21.000 cuft) each are fed by the excavator of the bedding system. The mills which have a diameter of 4,4 m (14,5 ft) and a length of 14 m (46 ft) are divided into two chambers a drying chamber of about 3 m (10 ft) and a grinding chamber of 10,5 m (34,5 ft). The partition wall is provided with blades, which transport the ore from the drying chamber to the grinding chamber.
Results with various mixtures
Acid mixtures. Except for one bed load of self fluxing pellets in the plant only acid pellets with a basicity of 0,15 have been produced.
The start up mixture A consisting of equal portions of five ores, one earthy, two magnetite, two hematite, had been chosen for ease of processing. Due to the magnetite content good strong pellets could be obtained with firing gas temperatures ranging from 1380 to 1280° C (2520 – 2310° F).
Although pellets fired at only 1200° C (2190° F) would be very weak, their metallurgical behaviour during reduction, the so called swelling was still correct.
Incorporating the earthy ore also made this mixture only mildly sensitive for fluctuations in moisture and bentonite addition.
As delivery of some of the ores of mixture A was impaired by strikes after two months of operation, we changed to mixture B, which has become our standard mixture. It consists of 30% magnetite, 20% earthy ores, 30% hematite concentrate, 20% hematite fines (total magnetite content 45%). Though somewhat more sensitive than mixture A plant operation with it is very easy.
Several deviations from the standard mixture have been applied in plant operation.
The magnetite content of the mixture has been varied in the steps 55%- 45% – 33% – 20%. As the external heat to be supplied for obtaining the indurating temperature of roughly 1300° C increases in the series 0,18 – 0,20 – 0,22 – 0,25 Gcal/ton, the retention time in the firing zone must, be increased at least proportionally.
Our firm believes that it should strive to have some simulation available for all processes in the main chain of operations. In the ore preparation field this ensures flexibility in the choice of raw materials, assessing the performance of those in the sintering or pelletizing processes and the quality of their product for blast furnace use.