Effect of rock properties on rippability of laterite in Iron Ore mines of Goa

The iron ore deposits in Goa state are mainly hematite and extend in North-North West, South-South East directions from North to the South occupying nearly one-fifth area of the total territory. The iron ore deposits of Goa can be broadly divided into three sectors; Northern Sector (Area to the north of river Madei), Central Sector (Area between the river Madei in the north and Sanvordem railway station in the south), Southern Sector (Area to the south of sanvordem railway station). To understand rippability three iron ore mines were chosen each from northern, central and southern sector as shown in Figure 1. All three iron ore mines are operated by open pit mining method and managed by private companies. Specifications of ripper dozer observed for study are shown in Table 1.

Table 1. Ripper Dozer Machine specifications

First mine “A” is from the northern sector, situated in Bicholim Taluka, North Goa. Ripper dozer operations were observed at four different places within the mine. Second mine “B” is from central section, and in Dharbandora Taluka, South

Goa. Ripper dozer operations were observed at four different places within the mine. Third mine “C” is from the southern sector, situated at Sanguem & Quepem taluka, South Goa, Goa.

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Figure 1. Location map showing three study Sites [2]

Ripper dozer operations were again observed at four different places within the mine. All studied mines are covered by rocks of the Goa Group of Dharwar Super Group, Archaean-Proterozoic age and stratigraphic succession of the rocks is shown in Table 2.

Iron Ore in Goa was formed from banded hematite quartzite’s and ferruginous phyllites through a specific process which cannot be attributed, the process of leaching away silica, replacement by iron concentration [3].

Table 2. The stratigraphic succession of the rocks of Goa region [3]

Location, general description and regional geology of all three mines are explained in paragraphs below.

Mine A: This mine belongs to Bicholim-Sirigao deposit. The rock in this area comes under the Goa group of rocks of the Dharwar super group. This deposit is situated between Bicholim-Dobdobo in the south-east and Sirigao in North-west. The iron ore deposits are enclosed within pink phyllites and manganiferous clay lenses. The whole unit is capped by hard laterite and lumpy laterite on the surface followed beneath by friable ore and soft phyllitic and manganiferous clays. Lenses of intercalated clays are also present within friable ore body. Further below the water table friable ore is continued as BHQ and BMQ sandwiched between quartz chlorite schist. Dolomite is found deep and beneath it there is no mineralization. The iron ore deposits are strata bound deposits wherein the concentration of iron ore is mainly because of weathering, the principal processes being supergene enrichment by leaching of silicates and other associated gangue from banded ferruginous quartzite.

Mine B: Most of lease area is covered by laterite and except in low lying areas having a soil cover. The laterite exposures have hematitic/magnetic/martite and limonitic ore with ore pieces. Ore body strikes almost in N 90°- N 270° with medium to steep Northerly and Southerly dips. The main constituents of exposed rocks are oxides, and hydrated oxides of iron, silica, and aluminum hydroxide.

The ore body is associated with phyllitic clays and limonitic clays along bands of Manganiferous clays within the normal ore body. Banded Hematite/Magnetite quartzite is on the foot wall side. The over-burden and side-burden generally consist of varying thickness of laterite, enriched lumpy laterite, phyllitic & manganiferous clays and altered intrusive rocks / clays.

Mine C: In Mine C, two ore bands are present. Both bands have laterite, lumpy laterite ore outcrops and form gently dipping synclinal basins with siliceous manganiferous clay as the footwall.  The thickness of the ore body is around 30 m on an average and is interspersed with phyllite intercalations.  The formations are traversed by igneous intrusions showing both concordant and discordant relations. The intrusives have been altered to clay and show sharp contacts with ore.  

The geological succession is seen in pit exposures and confirmed by core drilling data; Laterite 3m – 10 m, Lumpy Iron Ore 2 m – 15 m, Phyllite/Limonitic clay 3 m – 30 m, Powdery Ore (blue dust) 4m – 50 m and Manganiferous Clay as Footwall.

Field study, including investigations of rock material, in terms of location, rock type, depth of excavation is provided in Table 4. Different rock type as mentioned in Table 3 has been characterized in the laboratory as per ISRM 2008 standards.

Table 4. Field information of in-situ rock properties, ripper production performance and rippability observed at studied site

Ripper production was calculated based on volume by cross-sectioning method [4] in which area is first cross-sectioned, then time spent during ripping is recorded. After removing the ripped material, the ripped area was cross-sectioned again and the volume of ripped material was calculated. The ratio between the ripped volume and the ripping time gives the hourly production as shown in Table 4. The shape of the ripped cross section area assumed to be triangular and measurements were taken for average width of cut (W), average depth of cut (D) and cut length (L) to estimate ripper production as shown in Figure 3. The hourly ripper production (m³/hr) is calculated from the data and shown in Table 4. using the following formula [5];

$Q_{r}=\frac{\mathbf{q}_{\mathbf{r}} \times 60 \times \mathbf{E}_{\mathbf{r}}}{\mathbf{C}_{\mathbf{r}}}$   (1)

where; Qr = Ripper Production (m³/hr), q_r: production during a cycle (m³),

C_r = Cycle time in minute, (ripping and manoeuvring time), and E_r = Operators efficiency (90%)

Performed tests were uniaxial compressive strength (UCS) and young’s modulus as per ISRM 1979, indirect tensile strength (ITS) as per ISRM 1978, point load strength as per ISRM 1985, density determination as per ISRM 1977, and determination of moisture content as per ISRM 1977, slake durability as per ISRM 1972, sonic P-wave velocity of intact rock samples was determined by Sonic Viewer 170 equipment of OYO International, Japan, cerchar abrasivity test as per ISRM 2013. Three to four tests were performed for a particular rock type to arrive at an average test value. Among performed tests, few test results were utilized to understand the effect of rock properties on rippability of lateritic rocks containing iron ore as shown in Table 5.

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Figure 3. Simplified Geometry of ripping cut for volume calculation [4]

Table 5. Intact Rock properties tested in laboratory

Mine	Location in mine	Rock type	Samples	UCS* (MPa)	Samples	Young’s Modulus MPa)	Samples	PLS, (MPa)	Samples	ITS (MPa)	Samples	Sonic P-wave velocity (m/s )	Samples	CAI	Samples	Density, g/cc		MC %	Slake Durability%
																In-situ	Dry		First  cycle	Second cycle
A	A1	ILT	4	15.09	4	2065.62	4	2.65	4	2.08	4	2510	2	1.40	4	1.74	1.70	2.16	90.25	88.10
	A2	LT	4	9.37	4	934.41	4	1.9	4	1.50	4	2220	2	1.0	4	2.30	2.22	3.94	82.42	79.84
	A3	LTC	4	5.04	4	380.84	4	0.90	4	0.86	4	1440	2	0.45	4	1.86	1.78	4.76	73.27	69.84
	A4	LLT	4	22.65	4	3384.73	4	3.35	4	2.82	4	2990	2	2.55	4	3.20	3.15	1.74	87.29	86.13
B	B1	ILT	4	12.18	4	1868.58	4	2.21	4	1.66	4	2210	2	1.35	4	1.73	1.70	1.97	85.08	83.83
	B2	LT	4	9.89	4	983.43	4	1.7	4	1.31	4	1840	2	0.95	4	1.94	1.88	3.29	78.87	74.26
	B3	LTC	4	6.40	4	572.28	4	0.6	4	0.77	4	1250	2	0.55	4	1.72	1.66	3.93	72.44	67.08
	B4	LLT	4	19.23	4	2385.17	4	2.8	4	2.35	4	3115	2	2.25	4	3.12	3.06	1.79	89.51	87.24
C	C1	ILT	4	11.11	4	1485.20	4	2.1	4	1.43	4	1640	2	1.25	4	1.67	1.64	1.93	83.52	83.12
	C2	LT	4	8.08	4	827.62	4	1.5	4	1.10	4	1980	2	0.90	4	1.76	1.71	3.02	75.69	71.33
	C3	LTC	4	6.43	4	391.84	4	0.5	4	0.82	4	1260	2	0.40	4	1.58	1.53	3.50	69.87	63.10
	C4	LLT	4	17.81	4	2186.44	4	2.55	4	1.97	4	2810	2	2.10	4	2.92	2.87	1.85	85.81	81.22
ILT- Indurated Laterite, LT- Laterite, LTC- Laterite-Clay, LLT- Lumpy Laterite, UCS- Uniaxial compressive strength, PLS- Point load strength, ITS-Indirect tensile strength, CAI – Cerchar abrasivity Index MC- Moisture Content *[2].