ABSTRACT:

Cemented rockfill pillars are an integral part of the support system for the extraction of remnant (secondary) pillars at the Pyhasalmi mine. The costs associated with cemented backfilling are best reduced by the optimization of mix design and thorough dimensioning. The stability analysis (strength requirements) of a cemented rockfill pillar was performed with FLAC3D. The properties of the backfill was partly based on the results of laboratory tests.

Resume:

Des piliers de remblai rocheux cimintes font partie integrante du systeme de soutien pour I'extraction de piliers restants (secondaires) à la mine de Pyhasalmi, Les coûts associes au remblayage cimente sont mieux reduits en optimisant la conception du melange et en faisant un dimensionnement minutieux. L'analyse de stabilite (forces requises) d'un pilier de remblai rocheux ciminte à ete realisee avec FLAC3D. Les proprietes du remblai ont ete partiellement bases sur les resultants de tests en laboratoire.

ZUSAMMENFASSUNG:

Zementierte felsgefuellte Stuetzpfeiler sind ein integraler Bestandteil des Stuetzsystems zur Entfernung von ueberzahligen (sekundaren) Stuetzpfeilern in der Pyhasalmi Grube. Die Kosten, die man mit einer zementierten Rueckfuellung assoziiert, werden am besten durch eine Optimierung des Mischungsentwurfs und sorgfaltiger Dimensionierung reduziert. Die Stabilitatsanalyse (Stabilitatserfordernisse) eines zementierten feksgefuellten Stuetzpfeilers wurde mit FLAC3D durchgefuehrt. Die Eigenschaften der Rueckfuellung basierte teilweise auf den Resultaten von Labortests.

1 INTRODUCTION

The Pyhasalmi mine of Outokumpu Finnmines Oy is located in central Finland about 450 km north of Helsinki. Production started in 1962 with open pit mining and in 1967 underground mining was started. Over 1 million tons of low-grade massive sulphide ore is mined annually, averaging 0.8 per cent copper, 2.2 per cent zinc and 39 per cent sulphur. The ore is Contained within felsic and mafic metavolcanic rocks, and the orebody dips almost vertically. The main side rock of the ore, sericite schist, contains mainly quartz and sericite. The ore is stiff and good in strength but the sericite schist is weak. The maximum horizontal virgin stress below the +400 level is 40–70 MPa, approximately three times the vertical gravity load (Lappalainen et at. 1987).

2 BACKFILLING

Cemented rockfill pillars are an integral part of the support system for the extraction of remnant (secondary) pillars at the Pyhasalmi mine. The excavation takes place from the bottom upwards. The stopes are small, 20–30 m high and about 30 m wide, containing only 40 000 tons of ore on average. After the excavation, each stope is backfilled; the primary stopes are filled with rockfill stabilized by hydraulically placed cemented tailings containing water-granulated blastfurnace slag as a binder. In the second stage, the ore pillars are excavated and backfilled with loose rockfill. The Pyhasalmi Mine needs about 250 000 m3 of backfill material per year. In the cemented hydraulic backfill 50 000 m3 of classified tailings are used. About 40 000 m3 of rockfill comes from the underground development drifts and the remainder is from a surface quarry. The flotation tailings at the mine form less than 30 % of the mill feed. The tailings are classified by cyclones and the underflow fractions are taken to the backfill plant (Figure 1). The blastfurnace slag is mixed with classified tailings and activated with lime Ca(OH)2. This cemented part of the backfill is transported from the surface to the stopes through 76 mm plastic pipes and through rock holes.

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