New Dredge Concepts and Designs for Placer Mining Applications
Authors: Woolsey J.R. , O&apos;Hara C.G. 
Published: October 2008

The rigorous working conditions confronting the dredge mining industry in higher latitude offshore locations of recent interest exceed the capabilities of the conventional bucket-ladder dredge; the traditional ork horse of the industry. New concepts and designs developed by the Marine Minerals Technology Center (MMTC) offer solutions to these challenges. One, a hybrid suction system (conventional pump with jet pump assist) fitted with a motor drive, continuous, cutter/grizzly chain which sweeps the intake mouth clear of oversize rocks, removes same from diggingface, cuts and force feeds material into intake mouth and drives side cutter wheels for lateral cutting and directing material to intake.

A second system, the continuous bucket dragline, incorporates the flexibility of the dragline and efficiency of the continuous bucket line ladder dredge without the limitations of the ladder. The hybrid suction cutter chain/wheel design (CCW) is suitable for mounting on a low profile surface platform for increased stability in high latitude seas or on submersible bottom crawlers capable of operating beneath ice, in rough seas, or at greater than the traditional limits of the bucket-ladder system (greater than 15 m).

The continuous bucket dragline design (CBD) offers similar solutions under similar conditions as well as significant capital and operating cost advantages in conventional mining applications.

A prototype CCW dredge was designed and operated by the author for the purpose of evaluating submerged gold-bearing beach placers. The placers consisting of reworked bedrock and glacial material, ranging in size from clay to boulders, 2 to 4 meters in thickness, were successfully cut with transverse trenches. A prototype CBD dredge is currently in the planning stage in cooperation with a Mississippi gravel producer. The prototype CCW system provided a means of solving some of the fundamental problems which have hindered its application in mining heavy placer ground by 1) providing means of efficiently clearing the intake of oversize to prevent jamming; 2) transporting oversize away from the digging face to prevent the build-up of an impenetrable pavement; 3) force feeding desired ore material into high velocity slurry entrainment to insure heavy mineral pick-up. With these problems solved a workable system is now available for adaptation to low profile surface mounted systems or bottom crawlers.

The CBD promises to be equally versatile with added economic advantages for both new and traditional applications.

Introduction

In recent years there has been an increase in interest and activity in marine placer mining. Formerly offshore mining was mainly limited to the dredging of tin in Southeast Asia, iron sands in Japan and aggregate in the United Kingdom and Japan.

With the reported successful working of the offshore Nome gold placers, Norton Sound, Alaska, new focus has been directed at marine placer mining economics and technology. This study reviews both the favorable and unfavorable aspects of various traditionaldredge mining systems in offshore placer applications and discusses alternative conceptual designs to meet the rigorous requirements of the deeper locations, and the rough seas and ice condition! of the higher latitudes.