2,6 Minnesota Academy of Science 



Before leaving central Sweden mention should be made of the 

 Grangesberg iron ores. They occur in the usual formation of gneis- 

 sic rocks, called granulites or halleflinta gneisses or leptites. In- 

 cluded in this formation are also amphibolitic or dioritic green- 

 stones, skarn rocks, crystalline limestones and iron ores. Later 

 granitic and diabasic eruptives are also present. 



The ores are (a) apatitic; (b) quartzose hematites; (c) skarn 

 ores, and (d) calcareous ores; the latter two as well as the first 

 being non-manganiferous, non-titaniferous magnetites. The iron con- 

 tent varies from 55% to 65%; and the phosphorus from 0.1% to 

 8%. Thus the mineral apatite sometimes amounts to more than 

 40% of the ore. 



The low grade magnetite ores of central Sweden are now being 

 concentrated on a considerable scale. The process most favored 

 in that country is called the Grondal process from its inventor. 



The raw ore containing from 27% to 55% iron is broken in a 

 crusher, and ground to sand in a ball mill. The magnetite sand or 

 concentrate, containing 67% to 71% iron, is taken out by magnetic 

 separation, moistened and moulded into bricks about 6 inches 

 square and 2y 2 inches thick. No binder is used. The bricks are 

 simply pressed and moved slowly on iron conveyors through a fur- 

 nace heated with generator and furnace gases and subjected to an 

 oxidizing flame at the temperature of about 1400° C, which is 

 above the sintering heat. In these furnaces the magnetite is changed 

 to hematite, the percentage of sulphur is lowered, and a hard por- 

 ous briquet is made very suitable for the blast furnace. By this 

 process, at a cost of about eighty cents per ton, some twenty-seven 

 Grondal furnaces are turning out about 300,000 tons of briquets per 

 annum. Since magnetite briquets require about 300 pounds of char- 

 coal per ton of pig iron more than those made of hematite, the 

 expense of the briquetting is justified in a region of sulphurous 

 magnetite ores. This process is of special interest in Minnesota 

 because of the large quantity of low grade magnetite ores on the 

 eastern end of the Mesabi range, at present unmarketable. 



There remains for description in central Sweden what is his- 

 torically one of the most remarkable mines in the world, viz: the 

 Falun copper mine. Worked without interruption for the past six 

 hundred and fifty years, it had down to the end of the 19th century 

 produced more copper than any other mine on the globe. From 

 1630 complete records of its production are extant, according to 

 which the total output in that time amounts to nearly 300,000 tons, 

 or 600,000,000 pounds of copper. From the commencement of min- 

 ing operations to the present time its output has been estimated at 

 500,000 tons of copper, one ton of gold and fifteen tons of silver, 

 having an aggregate value of $250,000,000. For the sake of com- 

 parison we may note that in these days of monumental perform- 

 ances the total estimated copper output of the Falun mine in six 

 hundred and fifty years is about equal to three years' production of 

 the mines of Butte. 



The ore was formerly richer than in later years, and the pos- 

 sibility of secondary enrichment is again suggested. At present the 



