Problem with While mysqli_fetch_array

I have the following problem, I do not know how to do a while in codeigniter for the following example (this was done in object-oriented php, in a class, and I called it in a listboxt in the design)

function lista_sucursales(){

 $opciones="";
 $sql="SELECT * FROM sucursales";
 $resultado=mysqli_query($this->conexion,$sql);

 while($datos=mysqli_fetch_array($resultado)) {

    $id_suc=$datos["id_suc"];
    $nombre_suc=$datos["nombre_suc"];
    $opciones.="<option value='$id_suc'>$nombre_suc</option>";

  }
    return ($opciones);
 }

I'm trying to do it in a model but it does not work (I know it's wrong, but I do not really know how to do it ...)

 <?php

 class mHorarios extends CI_Model
 {

  function __construct()
  {
   parent::__construct();
  }

  public function lista_jc ()
  {

  $opciones="";


  $this->db->select('rut_usu,pnombre,apellido_pa,apellido_ma');
  $this->db->from('usuarios');


  while($datos=mysqli_fetch_array($this->load->database)) {

  $rut_usu=$datos["rut_usu"];
  $pnombre=$datos["pnombre"];
  $apellido_pa=$datos["apellido_pa"];
  $apellido_ma=$datos["apellido_ma"];

  $opciones.="<option value='$rut_usu'>$pnombre." ".$apellido_pa." 
  ".$apellido_ma</option>";

   }
   return ($opciones);


  }


 }

 ?>