Lets begin with fine ore storage bin or stockpile, get it out of the way and then proceed to grinding. You would think that there isnt that much to learn about when talking of fine ore storage, after all, all that this area is really for is to provide surge capacity for each of the circuits. True, but there are a couple of things that operators should know about. To begin with, I would like to explain to you what surge capacity is.
SURGE CAPACITY is an area that holds a minimum amount of ore to allow independent operating or an unbalanced volume of production for which ever circuits are involved. In this case it is crushing and milling. There are times when it is desirable to have either one or the other of these departments shut-down while the other department is operating.
This may he when there is mechanical maintenance being done or heaven forbid an operating error, surge capacity allows one circuit or the other to be shut down for a limited period of time without affecting the rest of the plants operation.
The design of a bin or pile is very basic, the ore is put onto the storage area from the top usually by a conveyor. The removal of the ore is from the bottom by more conveyors. There are a few factors that go into the design and selection of the type and size of a storage area however, the first is weather conditions.
Will the ore have to be protected from the elements such as rain and snow? What is the size of the milling department? The holding area has to be large enough to allow the mill ample operating time. This usually is between sixteen to forty-eight hours depending on the difference in tonnage between the through put of the crushing department and the through put of the milling department. In many mines especially the smaller ones the crushers are big enough to crush enough ore for the grinding circuit to run through a weekend without having to have an operating crew in to run the crushers.
Now that we know what the bin or pile is for, some of the problems that are associated with them are, TRAMP GARBAGE, FROZEN ORE, (these two are explained in detail in other segments) CLASSIFICATION, and of course the ever present SAFETY HAZARDS.
Any time that you have an ore movement the ore automatically classifies. It begins on the conveyor belt as it is taken to the fine ore bin. The shaking and vibration of the belt makes the finer particles of ore settle in to the spaces between the larger ore until the majority of the smaller rock is on the bottom of the load, near the surface of the conveyor belt. The trajectory of the ore as it leaves the conveyor belt will determine where in the bin or pile each size of the crushed ore will land.
Once in the bin, another stage of classification takes place. The smaller ore due to its lesser size and weight, tends to filter downwards faster than the coarse material does. This results in two different ore flows happening. The larger slow material and the smaller faster ore. To complicate this process, the ore is being pulled from the DRAW POINTS, these are the feeders that control the rate of flow of the ore as it leaves the storage area. What this does is guide the ore to these points. Much like the current you would find in a pond where there is a discharge creek. The maximum amount of movement is directly in front of the exit point. The effect of the current is lessened as you go to the side or get farther away from this point, the current is much the same within the fine ore storage area. The difference here is the faster settling fine material is being pulled toward the exit first. This means that as the bin is emptied there will be less and less fine material in the feed. This causes problems in the next stage of processing due to the larger rock interfering with the effectiveness of the process.
On the positive side, the fact that the feed does become coarse, is an early warning indication that the fine ore storage is almost empty and that closer monitoring of the feed will be required until the normal ore flow has resumed.
I mentioned earlier that tramp garbage was a problem, if steel, wire or wood is accidentally put into the storage area the problems that may develop are belts being ripped and chutes or screens plugged. This doesnt mean that all metal, wood, mine trash will cause damage, just the pieces that are too big to fit through feeders or are shaped in such a way that it is possible to cause a restriction in ore flow. In either case it may be expensive for the operation in both equipment and lost production. This is one of the main things that an operator will be on the lookout for when they are doing their circuit checks. That and of course any safety hazards that may be present.
I need to furthercomment on the influence of surge storage upon plant operation. The provision of adequate surge storage at the right places in the flow line is of sufficient importance to warrant a little further consideration.
It is desirable, in the crushing plant, as it is in any production process, to achieve a smooth and uninterrupted flow through the plant so that the reduction crushers and sizing screens will be operating at all times under uniform load conditions, and at full capacity. It is not always possible to feed the plant in this manner; a smooth flow of feed to the primary crusher is the exception, rather than the rule; therefore, if we are to smooth out the flow in the plant itself we must have elasticity in our flow line, and the logical way to obtain it is to provide surge storage ahead of each crushing stage. We can, of course, get around this by providing enough crushing capacity to absorb the peaks, but this method is expensive p. also it throws a variable load on-the screens; and screens do not perform at maximum efficiency under such a load.
While the case for an ample surge storage is clear cut enough, the question of just what constitutes an adequate storage is not always an easy one to answer. It is difficult to visualize beforehand just how smoothly a projected scheme of quarry or pit operation is going to function, and impossible to forecast accidental delays in loading, transportation, and feeding. About all we can do is torough out our plan of operation, and allow a comfortable margin for contingencies.
For example, if it is calculated that one trainload if rock will be de live red to the plant each one-half hour, on the average, and our primary crusher has been chosen with a view to getting this tram through in ten minutes, then we must, enviously. either provide stage crushing to match the high capacity of the primary or, if we want to get the most work out of our investment dollar, set up a surge capacity in the plant to absorb the peak load and feed it through the plant steadily over the entire 30 minute cycle. That is the essence of the problem: to keen as many as possible of the plant units rummy under uniform mad and at full capacity.
As a a example of one extreme in the need for surge capacity we have those plants whose primary and often secondary units operate only one shift of 8 or 10 hours/day, whereas the re-crushing and screening units run full time, or nearly so. The problem of calculating surge storage for such an operation is a simple one if operating conditions, such as character of feed and product size, remain unchanged, and if the feed rate to the primary unit is fairly constant from shift to shift. Where all of these conditions a re sub yet to significant variation the calculation is a more complicated one: we must calculate the need for each combination of conditions, and provide a surge storage that will take care of the maximum.
Regardless of how extreme the requirement for surge capacity may be it is usually unnecessary to provide an extensive storage ahead of more than one crushing stage. If the plant units are property balanced the :low rate can be smoothed out by regulating the feed from this one point. This does not necessarily mean that mechanical feeders need be used to regulate the flow; the crushers, will do this for the succeeding stages of the plant if the stage at which the regulating is done is the bottle-neck in the how line, i.e., the t.age of least capacity. Naturally, this regulating stage should be asnear the head end of the How line as possible, and, if it were possible, or practicable, to achieve an idea! plant layout, each successive stage would have a capacity slightly in excess of the preceding stage.
Any multi-stage crushing plant will have a bottle-neck stage, and it is this stage which establishes the production rate of the entire plant; therefore, it should be kept busy, and theonly way this can be done, if thefeed to the plant is a fluctuating one, is to provide sufficient surge storage ahead of it to absorb such fluctuations, at least, such as may be exported under normal operating conditions.
We have mentioned elsewhere that some operators of gravel plants have realized the desirability of incorporating in their flow line a surge storage of sufficient proportions to compensate for delays of considerable duration, such as might be caused by a major breakdown of the loading equipment. There is no reason why this same expedient cannot be applied in a quarry operation; following the primary, or secondary, crushing stage. Aside from the production insurance it affords, such a stockpile is an excellent point from which to regulate the feed to the reduction crushing stages and screens, by means of one or more mechanical feeders. With such regulation the need for surge storage at other points in the how line is minimized.
Already,I have discussed only that type of surge storage which lies directly in the flow line, a series storage, as if were. There is another type which can be utilized to advantage in certain operations, particularly in those gravel-pit operations where the gradation of the pit-run material is subject to considerable variation. This type of storage might be called the bleeder system, inasmuch as it consists of drawing off an intermediate size, or sizes, of product the amount of which, at any time, runs in excess of the capacity of the reduction crushers. Then, during periods when the pit-run material is running to tines, this storage is drawn upon to keep the reduction crushers busy. This is an excellent system for smoothing out plant production, especially in those plants which intermittently run heavily to the production of fine sizes of crushed rock. It is difficult to forecast the requirements for such a system: the application can best be made in a going plant where the operator mows his pit and his requirements for certain sizes of product.
Iron ore prices have risen to record highs in recent weeks, largely because supply has not been able to keep pace with demand in China, where crude steel production has grown by 30% over the past five years.
Expectations are building that benchmark prices can get to $200 a tonne topping the record $194 hit more than a decade ago as Chinese steelmakers ramp up production in defiance of government attempts to rein in output to control the industrys carbon emissions.
At the heart of spot iron ores 14% climb last month, helping drive thesuperchargedcommodities rally has been rising steel prices, from Asia to North America. Particular focus has been on China, where the economy has boomed and a swath of measures aimed at cleaning up the worlds biggest steel industry pushed mill profitability to the highest in more than a decade.
Citigroup expects benchmark prices to hit $200 within weeks. There will be a deficit of 18 million tonnes during the first three quarters of 2021 amid improved global steel demand and a slight miss in top miners shipments. The bank had previously predicted a 1 million tonne surplus.
If China wants to slow steel production, it needs to temper domestic demand. We expect to see additional measures from the government aimed at cooling steel demand, especially in the property sector, and that will likely be the catalyst for a correction in iron ore prices.
The China Iron and Steel Association said last week that fast-rising iron ore prices are unreasonable, that the industry should enhance the exploitation of resources both at home and abroad, and also improve rules for the futures market.
Analysts expect iron ore prices to ease over the course of the year, citing factors including Chinas steel-output measures taking effect and iron ore supply growth accelerating. Still, some market watchers have estimates for the second half of the year that are above $100 a tonne. Citigroup has forecast a drop to that level only under its most bearish scenario, while WoodMac says prices wont get below that threshold for another 12 months.
Steel prices could be slightly softer in the second half of 2021 on the back of some downstream resistance to higher prices and tighter credit conditions. This could see iron ore prices climb down from elevated levels of late April, S&P Global said in a note to investors.