The Arrastra was also one of the earliest crushing machines in use in America, being introduced at the same time as the Patio process—i.e., about 1557—and is still in wide use in Mexico, although chiefly in the treatment of silver ores by the Patio process. It is a circular, shallow, flat-bottomed pit, 10 to 20 feet in diameter, and paved with hard, uncut stones. Granite, basalt, and compact quartz are all used for the rock pavement, which is made 12 inches thick, and is either placed on a bed of well-puddled clay from 3 to 6 inches thick, or set in hydraulic cement, so that no chink or cranny remains into which the mercury or amalgam can find its way. In the centre a vertical shaft revolves, carrying two or four horizontal arms, to each of which is attached a heavy stone by thongs of bullock hide, or by chains. These drag stones weigh from 200 to 1,000 lbs. each, their forward ends being about 2 inches above the floor, whilst their other ends drag on it. They are moved by mules walking around outside the arrastra, or by water or steam-power, the speed varying from four to eighteen turns per minute. Fig. 21 represents an arrastra of the simplest description; at the front the stones forming the edge have been removed, so as to expose a section of the rock pavement.
Ore of about the size of pigeon’s eggs is introduced, enough water being added to make the pulp of the consistency of cream, and mercury is sprinkled over it after most of the grinding has been done. When the ore cannot be ground any finer, more water is added to dilute the pulp, the mule is driven slowly for half an hour to collect the mercury, and the pulp is then run out and another charge shovelled in. An arrastra of 10 feet in diameter takes a charge of 500 to 600 lbs. of ore, and treats about 1 ton per day of twenty-four hours. The amount of wear sustained by the grinding stones is equal to from 6 to 10 per cent, of the ore crushed. The output is so limited that the use of the arrastra has never been general outside Mexico, although it has been used in almost every district in the United States for a short time after the commencement of quartz mining in the particular locality.
It is often stated that the results obtained, so far as the percentage of gold extracted from refractory ores is concerned, cannot be equalled by any other amalgamating appliance, and that the Mexicans, using the arrastra, formerly treated at a profit ores which hardly yield any gold to the stamp mill, or even to the amalgamating pan. In consequence of its power of saving gold and the cheapness with which it can be erected and worked, the arrastra is still valuable for prospecting. In preparing the bed for this purpose, every care must be taken that the surface is even, and all joints properly cemented, or else the mercury and amalgam will in great part find their way into the foundations.
The reasons for the high extractive power of the arrastra, when treating certain ores, are no doubt to be found in the extreme fineness to which the ore is reduced, and in the prolonged contact between the ore and the mercury, which is maintained while they are being ground together. Moreover, the grinding action of the dragged stones keeps the particles of gold bright and in a suitable condition for amalgamation, without exercising force enough to flatten and harden them. The relative advantages of grinding surfaces consisting of iron and stone are less certain, and probably vary with the nature of the ore in course of treatment. An instance is given by Colonel Harris, in which stone was better than iron. In this case, at Cerro de Pasco, Peru, the old method of grinding ores in circular pans, by edge-runners of stone or granite, was found to entail a rapid wearing of the edge-runners, and, in order to remedy this, the runners were shod with iron. The returns at once fell off, and on careful trial it was found that the yield in the old machines was 15 per cent, more than in the new ones. Other similar instances are on record, but it must nevertheless be conceded that, with some ores, the presence of iron is necessary for good work, chlorides of silver and mercury being reduced by it, which would otherwise be lost in the tailings. The somewhat extravagant views often expressed, upholding the great superiority of the arrastra over its more modern rivals as an amalgamating machine for gold ores, are perhaps hardly justified, since a direct comparison with stamps or pans has been possible in only a few instances.
One of the most remarkable of these instances is that afforded by the experience at the Pestarena Mines, Yal Anzasca, Italy. The ore of this mine contains about 20 per cent, of pyrites, and is of somewhat low grade, rarely containing as much as 15 dwts. of gold per ton. Efforts were at first made to treat it by means of stamps and amalgamated plates, with the result that only 65 per cent, of the gold was extracted. Better results attended the introduction of the Francfort mill, a modified arrastra, driven in this instance by steam; this mill is substantially a wooden pan, with dies and shoes of stone. The mercury was added to the pulp after it had been finely ground, and the amalgamation and grinding of the pulp subsequently kept up for seven hours. From ore containing 12.3 dwts. per ton, the mill extracted 10.2 dwts., or 83 per cent., for a considerable period of time, while in the year 1890 the average extraction was 79.4 per cent. There were twenty-eight mills, each of which treated 1,200 lbs. of ore per day. The stone bed is said to last ten months, and the shoes from six to eight weeks, the total cost of treatment being very low.
At Smartsville, and in other places in California, the arrastra was applied to crushing and amalgamating hard cement-gravel obtained from a drift mine. At Smartsville the cement is coarsely crushed by being passed through a Grates rock-breaker, and is then charged into four arrastras, each of which is 12 feet in diameter and 3 feet deep and capable of containing from 5 to 9 tons of gravel. The grinding is effected by four blocks of diabase, each weighing from 600 to 1,000 lbs.; the rate of revolution is 14 times per minute, the time of grinding being one hour. A tablespoonful of mercury is fed in with each charge, and the total loss is only 10 per cent, of this. The pavement costs $40 and lasts for six months. At the end of the hour, a gate is opened in the side of the arrastra and the charge run out into a sluice, 200 feet long, where the mercury and amalgam are caught by means of riffles. The capacity of one arrastra is 50 tons of hard cement per day, or 75 to 90 tons of soft surface-gravel per day. The cost is from 6 to 8 cents per ton, and more gold is extracted than when a stamp battery was used at a cost of from 20 to 40 cents per ton. One man attends to the four arrastras, and another to the Gates crusher. Iron prospecting arrastras with stone drags are sometimes used.
There were about 100 arrastras at work in California in 1889, each treating from 1 to 3 tons of ore per day. They are used where only small quantities of high-grade ore are available for treatment.
The Arrastra is used in small gold mills to treat high grade gravity concentrates by batch amalgamation. Its capacity is 200 to 300 pounds of concentrates per day. The usual practice is to grind a small charge 5 to 7 hours, or until fine enough to add mercury, after which grinding is continued for several hours. Every day the finely ground charge is flushed out to waste and a new charge shovelled in. Periodically the accumulated amalgam is cleaned out of the Arrastra.
The Arrastra consists of two alloy iron shoes which are dragged along the bottom of a 30″ diameter circular bowl by means of a rotating arm. The bowl and bevel gear drive are mounted on cast iron legs. Power required is about 3 horsepower. Shipping weight on the belt-driven machine is approximately 750 pounds. Also see the Chilean Mill.
The Arrastra OR Chilean Edge Mill. Used commonly in Chilean gold ore processing, the edge mill has two large steel rimmed concrete wheels (these would have been stone in previous centuries) which roll around a circular concrete track and grind the gold ore beneath them. Final product size can be very small and the final size is a function of the time in the crusher. Grinding is usually carried out wet, the ore being washed in and out of the circular track by the water.