Bagaimana Batubara di Tambang
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Bagaimana batubara ditambang ?
Seperti yang terjadi pada 50 tahun lalu, sebagian besar batubara dihasilkan dari dua jenis utama yaitu tambang bawah tanah dan permukaan. Tetapi metode untuk recovering batubara dari bumi telah mengalami perubahan drastis dalam 25 tahun terakhir, sebagai konsekuensi dari kemajuan teknologi.
Lima puluh tahun yang lalu ketika pertambangan batubara sebagian besar dilakukan secara manual, tambang bawah tanah menyumbang 96 persen dari batubara yang dihasilkan setiap tahun. Saat ini, hampir 60 persen dihasilkan dari tambang permukaan. Selain itu metode tambang bawah air juga relatif baru yang dilakukan dibawah samudra.
Tambang Bawah Tanah (Underground Mine)
Tambang bawah tanah mengacu pada metode pengambilan bahan mineral yang dilakukan dengan membuat terowongan menuju lokasi mineral tersebut.
Berbagai macam logam bisa diambil melalui metode ini seperti emas, tembaga, seng, nikel, dan timbal.
Karena letak cadangan yang umumnya berada jauh dibawah tanah, jalan masuk perlu dibuat untuk mencapai lokasi cadangan. Jalan masuk dapat dibedakan menjadi beberapa macam:
- Ramp, jalan masuk ini berbentuk spiral atau melingkar mulai dari permukaan tanah menuju kedalaman yang dimaksud. Ramp biasanya digunakan untuk jalan kendaraan atau alat-alat berat menuju dan dari bawah tanah.
- Shaft, yang berupa lubang tegak (vertikal) yang digali dari permukaan menuju cadangan mineral. Shaft ini kemudian dipasangi semacam lift yang dapat difungsikan mengangkut orang, alat, atau bijih.
- Adit, yaitu terowongan mendatar (horisontal) yang umumnya dibuat disisi bukit atau pegunungan menuju ke lokasi bijih.
Ada dua tahap utama dalam metode tambang bawah tanah:
- development (pengembangan) dan
- production (produksi).
Sedang tahap production adalah pekerjaan menggali sumber bijih itu sendiri. Tempat bijih digali disebut stope (lombong). Disini uang mulai bisa dihasilkan.
Ada dua metode tambang bawah tanah:
- Tambang room and pillar
- dan Tambang longwall.
dimana batubara diambil dari pilar-pilar tersebut pada saat para penambang kembali ke atas. Atap tambang kemudian dibiarkan ambruk dan tambang
tersebut ditinggalkan.
Jenis metode nya sebagai berikut :
1. Open Stope
Open stope adalah salah satu metoda penambangan bawah tanah. Open Stope adalah penambangan tanpa membuat penyangga-penyangga. Syarat bahan galian yang dapat ditambang dengan metoda ini adalah atapnya cukup kuat menahan beban tanpa disangga atau dengan atau bisa disebut juga cukup kompeten.
2. Supported Stope
Dalam metoda penambangan seperti ini (Pada umumnya mineral logam) bawah tanah dengan cara membuat penyangga-penyangga. Dalam penyanggaan bahan yang bisa digunakn seperti kayu, besi, beton, atau baut besi (roof bolting).
3. Long Wall
Long Wall adalah suatu sistem penambangan bawah tanah untuk endapan batubara dengan membuat lorong-lorong panjang, secara mekanis dan bagian dari front penambangan yang sudah selesai ditambang dibiarkan runtuh dengan sendirinya (caving).
4. Short Wall
Short wall adalah penambangan bawah tanah untuk endapan batubara, dengan membuat lorong-lorong yang ukurannya lebih kecil atau lebih pendek dari long wall.
5. Room and Pillar
Room an d pillar merupakan suatu system penambangan bawah tanah untuk endapan batubara dengan menggunakan penyangga-penyangga yang umumnya dari kayu, dengan bentuk blok-blok persegi.
6. Cut and Fill
Cut and fill adalah salah satu metoda penambangan, dalam metoda penambangan ini, dengan cara menggali atau membuat bukaan-bukaan dan kemudian mengisi kembali dengan material lain bekas bukaan tersebut.
7. Gophering
Dalam metoda penambangan ini dengan membuat bukaan-bukaan berukuran relatif kecil dan sempit secara tidak beraturan, atau dikenal sebagai lobang tikus.
8. Block Caving
Merupakan suatu sistem penambangan bawah tanah, dengan car meruntukan bagian yang sudah selesai ditambang (mined out ).
Tambang bawah Air (Underwater Mine).
Penambangan bawah laut adalah proses pengambilan mineral yang relatif baru dilakukan di dasar samudra. Situs penambangan samudra biasanya berada di sekitar kawasan nodul polimetalik atau celah hidrotermal aktif dan berada pada kedalaman 1.400 - 3.700 meter di bawah permukaan laut. Celah tersebut menciptakan deposit sulfida, yang berisikan logam mulia seperti perak, emas, tembaga, mangan, kobalt, dan seng. Deposit tersebut ditambang menggunakan pompa hidrolik atau sistem ember yang mengangkut bijih ke permukaan untuk diproses. Mengenai operasi penambangan, penambangan bawah laut memunculkan pertanyaan mengenai kerusakan lingkungan terhadap daerah sekitar.
Tahap-tahap kegiatan Penambangan :
a. Penyelidikan Umum Kegiatan ini merupakan langkah awal usaha pertambangan yang ditujukan untuk mencari dan menemukan endapan bahan galian. Kegiatan penyelidikan umum dilakukan dengan tujuan mencari komoditas bahan galian tertentu maupun di lokasi tertentu, artinya penyelidikan hans difokuskan pada (tipe/jenis) bahan galian yang spesifik atau pada area yang spesifik (wilayah/Negara)dan mempelajari keadaan geologi secara umum untuk daerah yang bersangkutan berdasarkan data permukaan.
b. Eksplorasi Merupakan kegiatan lanjutan dari penyelidikan umum yang bertujuan untuk mendapatkan kepastian tentang endapan bahan galian tersebut yang meliputi bentuk, ukuran, letak kedudukan, kualitas (kadar) endapan bahan galian serta karakteristik fisik endapan bahan galian dan batuan samping.
c. Studi Kelayakan Merupakan tahapan akhir dari rentetan penyelidikan awal yang dilakukan sebelumnya sebagai penentu apakah kegiatan penambangan endapan bahan galian tersebut layak dilakukan atau tidak. Dasar pertimbangan yang digunakan meliputi pertimbangan teknis dan ekonomis dengan memperhatikan keselamatan kerja serta kelestarian lingkungan hidup.
d. Persiapan penambangan Adalah kegiatan ini meliputi penyiapan infrastruktur dan lahan kerja penambangan yang antara lain meliputi pembuatan jalan, pembabatan semak/pohon, penupasan tanah penutup, pembangunan kantor, gedung, bengkel, dll.
e. Penambangan Adalah kegiatan penambangan yang dimaksud adalah kegiatan yang ditujukan untuk membebaskan dan mengambil bahan galian dari dalam kulit bumi, kemudian dibawa ke permukaan untuk dimanfaatkan.
f. Pengolahan Bahan Galian Adalah kegiatan yang bertujuan untuk meningkatkan kadar atau mempertinggi mutu bahan galian yang dihasilkan dari tambang sampai memenuhi persyaratan untukdiperdagangkan atau sebagai bahan baku untuk industri lain.Keuntungan lain dari kegiatan ini adalah mengurangi jumlah volume dan beratnya sehingga dapat mengurangi ongkos pengangkutan.
g. Pengangkutan Adalah segala usaha untuk memindahkan bahan galian hasil tambang atau pengolahan dan pemurnian dari daerah penambangan atau tempat pengolahan dan pemurnian ke tempat pemasaran atau pemanfaatan selanjutnya dari bahan galian tersebut.
h. Pemasaran Adalah kegiatan untuk memperdagangkan atau menjual hasil-hasil penambangan dan pengolahan bahan galian.
i. Reklamasi Merupakan kegiatan untuk merehabilitasi kembali lingkungan yang telah rusak baik itu akibat penambangan atau kegiatan yang lainnya. Rehabilitasi ini dilakukan dengan cara penanaman kembali atau penghijauan suatu kawasan yang rusak akibat kegiatan penambangan tersebut
Tambang Terbuka (Open Pit Mine/ Surface Mining)
Tambang terbuka (surface mining) merupakan satu dari dua sistem penambangan yang dikenal, yaitu Tambang terbuka dan Tambang Bawah Tanah. dimana segala kegiatan atau aktivitas penambangan dilakukan di atas atau relatif dekat permukaan bumi dan tempat kerja berhubungan langsung dengan dunia luar.
Tambang terbuka (open pit mine) adalah bukaan yang dibuat di permukaan tanah, bertujuan untuk mengambil bijih dan akan dibiarkan tetap terbuka (tidak ditimbun kembali) selama pengambilan bijih masih berlangsung.
Untuk mencapai badan bijih yang umumnya terletak di kedalaman, diperlukan pengupasan tanah/batuan penutup (waste rock) dalam jumlah yang besar. Tujuan utama dari operasi penambangan adalah menambang dengan biaya serendah mungkin sehingga dicapai keuntungan yang maksimal.
Pemilihan berbagai parameter desain dan penjadwalan dalam pengambilan bijih dan pengupasan batuan penutup melibatkan pertimbangan teknik dan ekonomi yang rumit. Mesti diambil kompromi yang optimal antara memaksimalkan perhitungan ekonomis dan adanya parameter pembatas karena faktor geologi dan pertimbangan teknik lain.
Dengan berkembangnya teknologi dan teknik pertambangan, cadangan yang dulunya dinilai tidak ekonomis, sekarang dapat berubah menjadi sumber yang layak tambang. Hal ini juga didorong oleh meningkatnya permintaan akan bahan tambang seiring dengan peningkatan konsumsi per kapita.
Secara umum, tambang terbuka dinilai lebih menguntungkan dibanding metode tambang bawah tanah dalam hal recovery (mineral yang dapat ditambang dibanding dengan banyak cadangan), grade control (pengendalian kadar), keluwesan operasi, keselamatan, dan lingkungan kerja.
Namun, dalam situasi dimana deposit terlalu kecil, berbentuk tak teratur, atau terletak terlalu dalam di bawah tanah, metode tambang bawah tanah akan lebih menguntungkan.Suatu tambang terbuka pada satu titik mungkin saja perlu diubah menjadi tambang bawah tanah ketika batuan penutup (waste rock) yang perlu dikupas menjadi terlalu besar. Ini biasanya terjadi jika cadangan bijih berlanjut hingga sangat dalam.Faktor teknologi, kondisi pasar, dan kebijakan pemerintah akhirnya juga akan turut jadi pertimbangan dalam pemilihan metode tambang yang pas.
Penambangan pada tambang terbuka itu sendiri dilakukan dengan beberapa tahapan kerja : pengurusan surat-surat ijin yang dibutuhkan untuk kegiatan penambangan, pembabatan (land clearing), pengupasan lapisan tanah penutup (stripping of overburden), penambangan (exploitation), pemuatan (loading), pengangkutan (hauling), dan pengolahan serta pemasaran.
Tahap Penambangan :
- Prospeksi
- Persiapan
- Pembersihan lahan ( land clearing)
- Pengupasan Tanah Pucuk ( top soil)
- PenggalianTanah Penutup (overburden removal)
- Penambangan Batubara ( coal getting)
- Pengangkutan Batubara ke ( coal hauling)Backfilling (dari tempat
- penyimpanan sementara)
- Perataan dan Rehabilitasi Tanah ( spreading)
- Penghijauan (reclamation)
- Kontrol (monitoring)
1. Pengelompokan Tambang Terbuka
Pada prinsipnya tambang terbuka dapat digolongkan ke dalam empat golongan :
a. Open pit/Open mine/Open cut/Open cast.
Adalah tambang terbuka yang diterpakan pada penambangan ore (bijih). Misalnya nikel, tembaga, dan lain-lain.
b. Strip Mine.
Penerapan khusus endapan horizontal/sub-horizontal terutama untuk batubara, dapat juga endapan garam yang mendatar. Contoh Tamabang Batubara di Tanjung Enim.
c. Quarry
AdalahTambang terbuka yang diterapkan pada endapan mineral industri (industrial mineral). Contoh Tambang batu pualam di Tulung Agung.
d. Alluvial mining
Dapat dikatakan sebagai “placer Mining” ataupun di Australia disebut “Beach-mine” yaitu cara penambangan untuk endapan placer atau alluvial. Contoh tambang Cassiterite di Pulau Bangka, belitung dan sekitarnya.
2. Konsiderasi Pada Operasi Penambangan
Secara garis besar, faktor-faktor yang berpengaruh terhadap kelangsungan kegiatan penambangan dibagi dalam dua kategori, yaitu faktor teknis dan faktor ekonomi.
a. Kajian Secara Teknis
Unsur unsur teknis yang perlu mendapat perhatian dalam pelaksanaan aktifitas kegiatan kerja sebuah proyek penambangan meliputi :
1. Kondisi Umum tempat proyek dilaksanakan
Kondisi Kondisi tempat kerja yang perlu diperhatikan adalah meliputi kondisi geologi, topografi, iklim dan sosial Budaya. Keadaan umum tersebut mutlak diperhitungkan guna menentukan penjadwalan waktu kegiatan dan yang utama sekali menetapkan efesiensi kerja kerja efektif dari pelaksanaan proyek tersebut.
2. Sarana perlengkapan peralatan kerja
Jenis perlengkapan dan peralatan kerja disesuaikan dengan kondisi tempat kerja, maksud pekerjaaan, kapasitas produksi, dan efektifitas kerja yang diinginkan. Cara pengadaanya diperhitungkan dengan umur produksi dan efektifitas kerja dan ketersediaan modal kerja yang di miliki.
3. Metode Pelaksanaan kerja
Dalam proyek ini pelaksanaan kegiatan pembongkaran material dilakukan dengan peledakan. Metode tersebut dipilih mengingat jenis materialnya memilki kekerasan yang cukup tinggi, fraksi material yang lepas yang sasaran produksinya telah ditentukan.
b. Kajian Secara Ekonomis
Kajian secara ekonomis dimaksudkan untuk mengetahui sebuah proyek penambangan memperoleh keuntungan atau tidak. Dalam perhitungan aliran uang diperhatikan beberapa faktor yang berpengaruh dalam situasi ekonomi. Hal-hal yang diperhatikan tersebut adalah:
- Nilai (value) daripada endapan mineral per unit berat (P). dan biasanya dinyatakan dengan ($/ton) atau (Rp/ton).
- Ongkos produksi (C), yaitu ongkos yang diperlukan sampai mendapatkan produknya diluar ongkos stripping.
- Ongkos stripping of overburden (Cob).
- Cut Off Grade, akan menentukan batas-batas cadangan sehingga menentukan bentuk akhir penambangan.