Many attempts had been made in by-gone days to concentrate the iron in such ores by water processes, but with only a partial degree of success. The impossibility of obtaining a uniform concentrate was a most serious objection, had there not indeed been other difficulties which rendered this method commercially impracticable. It is quite natural, therefore, that the idea of magnetic separation should have occurred to many inventors. Thus we find numerous instances throughout the last century of experiments along this line; and particularly in the last forty or fifty years, during which various attempts have been made by others than Edison to perfect magnetic separation and bring it up to something like commercial practice. At the time he took up the matter, however, no one seems to have realized the full meaning of the tremendous problems involved.
From 1880 to 1885, while still very busy in the development of his electric-light system, Edison found opportunity to plan crushing and separating machinery. His first patent on the subject was applied for and issued early in 1880. He decided, after mature deliberation, that the magnetic separation of low-grade ores on a colossal scale at a low cost was the only practical way of supplying the furnace-man with a high quality of iron ore. It was his opinion that it was cheaper to quarry and concentrate lean ore in a big way than to attempt to mine, under adverse circumstances, limited bodies of high-grade ore. He appreciated fully the serious nature of the gigantic questions involved; and his plans were laid with a view to exercising the utmost economy in the design and operation of the plant in which he contemplated the automatic handling of many thousands of tons of material daily. It may be stated as broadly true that Edison engineered to handle immense masses of stuff automatically, while his predecessors aimed chiefly at close separation.
Reduced to its barest, crudest terms, the proposition of magnetic separation is simplicity itself. A piece of the ore (magnetite) may be reduced to powder and the ore particles separated therefrom by the help of a simple hand magnet. To elucidate the basic principle of Edison's method, let the crushed ore fall in a thin stream past such a magnet. The magnetic particles are attracted out of the straight line of the falling stream, and being heavy, gravitate inwardly and fall to one side of a partition placed below. The non-magnetic gangue descends in a straight line to the other side of the partition. Thus a complete separation is effected.
Simple though the principle appears, it was in its application to vast masses of material and in the solving of great engineering problems connected therewith that Edison's originality made itself manifest in the concentrating works that he established in New Jersey, early in the nineties. Not only did he develop thoroughly the refining of the crushed ore, so that after it had passed the four hundred and eighty magnets in the mill, the concentrates came out finally containing 91 to 93 per cent. of iron oxide, but he also devised collateral machinery, methods and processes all fundamental in their nature. These are too numerous to specify in detail, as they extended throughout the various ramifications of the plant, but the principal ones are worthy of mention, such as:
The giant rolls (for crushing).
Intermediate rolls.
Three-high rolls.
Giant cranes (215 feet long span).
Vertical dryer.
Belt conveyors.
Air separation.
Mechanical separation of phosphorus.
Briquetting.
That Mr. Edison's work was appreciated at the time is made evident by the following extract from an article describing the Edison plant, published in The Iron Age of October 28, 1897; in which, after mentioning his struggle with adverse conditions, it says: "There is very little that is showy, from the popular point of view, in the gigantic work which Mr. Edison has done during these years, but to those who are capable of grasping the difficulties encountered, Mr. Edison appears in the new light of a brilliant constructing engineer grappling with technical and commercial problems of the highest order. His genius as an inventor is revealed in many details of the great concentrating plant.... But to our mind, originality of the highest type as a constructor and designer appears in the bold way in which he sweeps aside accepted practice in this particular field and attains results not hitherto approached. He pursues methods in ore-dressing at which those who are trained in the usual practice may well stand aghast. But considering the special features of the problems to be solved, his methods will be accepted as those economically wise and expedient."
A cursory glance at these problems will reveal their import. Mountains must be reduced to dust; all this dust must be handled in detail, so to speak, and from it must be separated the fine particles of iron constituting only one-fourth or one-fifth of its mass; and then this iron-ore dust must be put into such shape that it could be commercially shipped and used. One of the most interesting and striking investigations made by Edison in this connection is worthy of note, and may be related in his own words: "I felt certain that there must be large bodies of magnetite in the East, which if crushed and concentrated would satisfy the wants of the Eastern furnaces for steel-making. Having determined to investigate the mountain regions of New Jersey, I constructed a very sensitive magnetic needle, which would dip toward the earth if brought over any considerable body of magnetic iron ore. One of my laboratory assistants went out with me and we visited many of the mines of New Jersey, but did not find deposits of any magnitude. One day, however, as we drove over a mountain range, not known as iron-bearing land, I was astonished to find that the needle was strongly attracted and remained so; thus indicating that the whole mountain was underlaid with vast bodies of magnetic ore.
"I knew it was a commercial problem to produce high-grade Bessemer ore from these deposits, and took steps to acquire a large amount of the property. I also planned a great magnetic survey of the East, and I believe it remains the most comprehensive of its kind yet performed. I had a number of men survey a strip reaching from Lower Canada to North Carolina. The only instrument we used was the special magnetic needle. We started in Lower Canada and travelled across the line of march twenty-five miles; then advanced south one thousand feet; then back across the line of march again twenty-five miles; then south another thousand feet, across again, and so on. Thus we advanced all the way to North Carolina, varying our cross-country march from two to twenty-five miles, according to geological formation. Our magnetic needle indicated the presence and richness of the invisible deposits of magnetic ore. We kept minute records of these indications, and when the survey was finished we had exact information of the deposits in every part of each State we had passed through. We also knew the width, length, and approximate depth of every one of these deposits, which were enormous.
"The amount of ore disclosed by this survey was simply fabulous. How much so may be judged from the fact that in the three thousand acres immediately surrounding the mills that I afterward established at Edison there were over 200,000,000 tons of low-grade ore. I also secured sixteen thousand acres in which the deposit was proportionately as large. These few acres alone contained sufficient ore to supply the whole United States iron trade, including exports, for seventy years."
Given a mountain of rock containing only one-fifth to one-fourth magnetic iron, the broad problem confronting Edison resolved itself into three distinct parts—first, to tear down the mountain bodily and grind it to powder; second, to extract from this powder the particles of iron mingled in its mass; and, third, to accomplish these results at a cost sufficiently low to give the product a commercial value.