Pebble Crushing in SAG Mill Circuit

Pebble Crushing in SAG Mill Circuit

Full Scale Test Circuit

Based upon the results of the pilot plant test work, a decision was made to conduct a full scale test in 1990 utilizing one of the ten (10) primary mill lines at the National Steel Pellet Plant. Primary Mill Line No. 10 would be converted to a fully autogenous grinding mill with pebble extraction and crushing. Engineering and installation of the test circuit was completed during the first half of 1990. Testing was conducted during a period of twenty (20) weeks in 1990 and fourteen (14) weeks in 1991.

The pebble crushing test circuit as originally designed extracted a minus 76 mm (-3 in.) product from the fully autogenous mill through 76 mm (3 in.) square discharge grates. A vibrating screen was used to provide a mill discharge product separation at 19 mm (¾ in.). The minus 76 trail (-3 in.) plus 19 mm (+¾ in.) screen oversize product, which are the “pebbles”, were then crushed to essentially 100% minus 19 mm (-¾ in.) in a crushing circuit. This product was returned to the feed end of the mill.

Final engineering for the test circuit was performed by a local consulting firm during the first three months of 1990. This included equipment layouts, general arrangement drawings, structural design, and detailed fabrication drawings. Engineering for electrical and instrumentation was performed in-house. The majority of the crushing circuit was located outside the existing concentrator adjacent to Primary Grinding Line No. 10.

1990 Test Run

During this test run, pilot plant projections were to be confirmed which indicated that the throughput of an autogenous grinding with pebble crushing circuit would be equal to or better than the throughput currently achieved in the existing SAG mill circuits. It was also planned to verify whether or not a significantly finer grind could be obtained in the autogenous test circuit as compared to the SAG mill circuits with only a minor loss in mill throughput.

Basically the test circuit did not initially perform as projected due to numerous causes. These included design limitations within the test circuit, incorrect sizing of the pebble port grates, inadequate control of feed size to the test mill, and control monitoring capabilities and philosophy that were not compatible with the requirements of an autogenous grinding circuit.

The initial pebble port grates had 448 each 76 mm by 76 mm (3 in. x 3 in.) square openings for a total open area of 2.6 square meters (4032 sq. in.). Soon after the start of the test it became apparent that there was not sufficient open area and/or the grate openings were too small to duplicate the pebble extraction projected from the pilot plant testing.

It was originally intended to make a separation on the mill discharge screen at 19 mm (¾ in.). The screen oversize was the crushing circuit feed and the screen undersize was pumped to Derrick screens that close circuited the mill.

As a result of these successful short runs with larger ore, areas in the pit that appeared to have coarser ore were selectively mined and this ore was supplied to the test circuit. Whenever the feed to the test mill increased in size, the throughput would increase.

The teed rate to the existing SAG mills at NSPC are controlled by mill power. A set-point for individual lines is established at from between 5000 KWH to 5450 KWH and the feed to the mill is automatically adjusted to maintain this power setting. This control philosophy resulted in a very erratic operation during the 1990 autogenous grinding test run. Substantial fluctuations were experienced in the circulating loads within the test circuit depending on the size of the feed reporting to the mill. S

To take advantage of periods when larger ore was available to the test circuit, the control of the feed to the mill was further changed by automating the mill feed rate based on feed size. This was made possible by an earlier decision to install an optical material size monitor on the primary mill feed belt. As the feed size increased the feed rate would be automatically increased, and as the feed size decreased the feed rate was decreased.

1991 Test Run

The original objectives for this additional test run were to further confirm the affects of ore size on mill performance, verify the operations ability to provide adequately sized feed, and to test additional circuit modifications in order to evaluate how a total plant installation would be affected. Much more was to be learned however, including the fact that the size of the crude ore as blasted in the pit was not the critical factor affecting the performance of the circuit.

The feed size provided to the test mill during the first two weeks of the 1991 test run was unacceptably small even though the specially designed coarse ore blast areas were being mined. Consequently, there was no measured improvement in the mill performance over the 1990 testing. The cause was attributed to new liners and mantle which had been installed in the primary crusher during the time between the 1990 and the 1991 test runs.

During the next three weeks, the test circuit production was noticed to be cyclical in nature, coinciding with swings in the feed size being furnished to the mill. For short periods of from 10 to 13 hours in length, acceptable throughputs of over 305 metric tons per hour (300 LTPH) would be achieved. These occurred as the crushed crude ore surge pile was approaching empty and the coarser ore sloughed from the sides of the pile. For the remainder of the period during each cycle, which lasted up to two days, the throughput remained unacceptably low at below 203 metric tons per hour (200 LTPH).

The throughputs achieved for the remainder of the test averaged in excess of 305 metric tons per hour (300 LTPH) and were consistently better than that of the existing SAG mill circuits. The 40% to 70% limits placed on the crude ore storage caused severe hardships, however, on the mining operation due to the frequent changes required in the mining operations to keep the test line in ore.

Additional testing was next performed to determine the actual effects of the modified blasting practices previously conducted to provide the “coarse ore” for the test circuit in view of the significant improvements achieved through the changes in the ore crushing and handling procedures. It was generally believed that a requirement for larger crude ore feed on a total plant conversion project would result in additional mining costs due to increased equipment wear and maintenance.

Data Collection

Data collection was initially provided for by automatically monitoring and recording the following key circuit operating parameters on an hourly basis:

  1. Primary Mill Feed Rate
  2. Crushing Circuit Feed Rate (Pebbles)
  3. Crushing Circuit Surge Bin Overflow Rate
  4. Crusher Product Rate (Crusher Throughput)
  5. Primary Mill Power
  6. Primary Mill Operating Time

Based upon the full scale test results, fully autogenous grinding with pebble extraction and crushing can feasibly be incorporated into the existing primary grinding circuits at the National Steel Pellet Plant. Throughputs from 330 metric tons to 353 metric tons per hour (325 to 347 LTPH) were verified to be achievable, which are approximately 10% greater than the present SAG mill operating mode.

sag mill circuit process flowsheet

sag mill circuit pebble crushing

sag mill circuit pebble crushing test

sag mill circuit general arrangement

sag mill circuit 1991 general arrangement

sag mill circuit test summary

pebble crushing test in a sag mill circuit